NRE Health & Wellbeing Matrix Library™ • Matrix 18

Cave & Underground Health & Wellbeing Matrix™

A comparative framework for understanding how enclosed geology, stone, darkness, natural acoustics, humidity, stable temperature, underground water, movement, clothing and footwear may influence opportunities for environmental interaction.

Developed by the NRE Health Institute • General edition • Version 1.0

Why caves and underground environments require a dedicated matrix

Caves and underground environments differ fundamentally from open-air natural settings. They may combine rock formations, low or absent natural light, stable temperatures, humidity, confined spaces, underground water, distinctive acoustics and reduced contact with ordinary surface conditions.

These environments vary substantially. A commercial show cave with lighting, railings and paved access does not provide the same interaction opportunity as a natural cave entrance, limestone chamber, lava tube, underground river or minimally developed cave system.

Underground depth, darkness and remoteness must also be separated from environmental quality. A deeper or more difficult cave is not automatically more beneficial, and increased exposure may create serious risks involving navigation, flooding, oxygen, instability and delayed rescue.

Assess underground environmental richness

Compare cave settings according to natural rock, geological formations, humidity, water, darkness, natural sound, ecological features and reduced built disturbance.

Recognise enclosed interaction

Account for stone, air temperature, acoustics, darkness, natural surfaces and underground water as distinctive forms of environmental interaction.

Separate immersion from technical risk

Recognise that a highly immersive cave may also require specialist lighting, helmets, footwear, route knowledge, gas monitoring and trained assistance.

Caves may provide unique environmental immersion through geology, darkness, sound, temperature and enclosed natural space, but greater depth and reduced infrastructure often require greater preparation, protection and technical competence.

What this matrix evaluates

The Cave & Underground Health & Wellbeing Matrix™ compares common cave and underground settings according to their relative opportunity for interaction with rock, geological formations, humidity, underground water, natural acoustics, darkness, stable temperature and movement through enclosed natural spaces.

It does not prescribe cave entry, underground swimming, climbing, crawling, barefoot use, reduced clothing or exploration beyond publicly approved and personally suitable areas.

Cave and underground settings

  • Artificial underground visitor space
  • Commercial show cave
  • Natural cave entrance
  • Shallow limestone cave
  • Lava tube
  • Large natural chamber
  • Underground river or water cave
  • Deep or technically demanding cave
  • Remote or minimally developed cave system
  • Lawful clothing-optional cave setting

Interaction variables

  • Natural rock and geological formations
  • Darkness and reduced artificial light
  • Stable underground temperature
  • Humidity and underground airflow
  • Natural acoustics and echo
  • Underground water and mineral surfaces
  • Movement through enclosed terrain
  • Geological and biological diversity
  • Built infrastructure and artificial lighting
  • Depth, confinement and route complexity
  • Flooding, gas and air-quality risk
  • Emergency access and rescue difficulty
  • Clothing and footwear as modifiers

Standardised conditions

Unless specifically stated otherwise, comparisons assume lawful access and conditions suitable for limited human activity, including stable rock, acceptable air quality, manageable humidity, safe water levels, reliable lighting, known routes, suitable temperature and available exit access. Flooding, rockfall, oxygen deficiency, toxic gas, unstable passages, severe confinement, contaminated water, closed caves and emergency conditions are excluded unless specifically identified.

Environmental classification does not measure cave difficulty

A cave may receive a high environmental class while remaining unsuitable for a particular person, route, clothing condition or activity. Passage size, rock stability, air quality, water depth, flooding, darkness, technical climbing, navigation and rescue access must be assessed separately from the environmental interaction grade.

Underground environments may become hazardous without warning

Rainfall outside the cave may cause rapid flooding underground. Lighting failure, disorientation, unstable rock, poor air quality, cold water and restricted exits may quickly create life-threatening conditions. Entry beyond managed areas may require specialist training, equipment and local knowledge.

Cave & Underground Health & Wellbeing Matrix™

This matrix compares common cave and underground environments according to their relative opportunity for interaction with natural rock, geological formations, darkness, humidity, underground airflow, water, acoustics, stable temperature, movement and reduced built disturbance.

Cave or underground environment Standard protective clothing Suitable footwear worn Standard protective clothing Footwear removed briefly where suitable Light clothing Reduced layers under controlled conditions Minimal clothing Environmental grade shown; restrictions may apply No clothing Environmental grade shown; restrictions may apply
Artificial underground visitor space Built tunnel, chamber or underground attraction with extensive artificial surfaces and lighting A Very limited A Very limited Public floors, debris and operational rules may require footwear B Limited B Limited Venue rules and thermal conditions may require clothing B Limited Nudity may not be lawful or permitted
Commercial show cave Natural cave formations with lighting, railings, stairs and managed visitor access C Moderate C Moderate Wet floors, stairs and operator rules may require footwear C Moderate C Moderate Visitor rules, temperature and public-access restrictions may apply C Moderate Nudity may not be lawful, permitted or thermally suitable
Natural cave entrance Daylight transition zone with rock, airflow, vegetation and partial enclosure C Moderate D High Sharp rock, animal waste and unstable surfaces may require footwear D High D High Public access, wildlife and temperature restrictions may apply D High Nudity may not be lawful, safe or appropriate
Shallow limestone cave Natural rock chamber with formations, humidity, darkness and limited route complexity D High D High Sharp rock, slippery surfaces and contamination may require footwear D High D High Cold surfaces and rock contact may require protective clothing D High Nudity may be unsafe, unlawful or environmentally inappropriate
Lava tube Volcanic passage with natural rock, darkness, distinctive acoustics and uneven terrain D High D High Sharp volcanic rock and unstable footing may make footwear essential D High Abrasion and temperature may require protective layers D High Minimal clothing may reduce protection from rough rock D High Nudity may be unsafe, unlawful or unsuitable
Large natural chamber Extensive enclosed space with natural formations, acoustics, humidity and limited artificial influence D High D High Wet rock, debris and uneven ground may require footwear D High D High Humidity, low temperature and rough surfaces may require clothing D High Nudity may not be lawful, safe or thermally suitable
Underground river or water cave Natural chamber or passage containing flowing or standing underground water D High Water, cold and slippery terrain may require specialist equipment D High Footwear removal may impair traction and mobility D High Cold water and wet surfaces may require thermal protection D High Minimal clothing may sharply increase cold and abrasion risk D High Nudity may be medically hazardous and may not be lawful
Deep or technically demanding cave Complex underground route involving crawling, climbing, water or restricted passages D High Specialist clothing, helmets, footwear and equipment may be essential D High Footwear removal may compromise safety and evacuation D High Reduced clothing may increase abrasion and thermal risk D High Minimal clothing is generally incompatible with technical cave safety D High Nudity may create serious injury, cold and rescue risks
Remote or minimally developed cave system Natural cave with limited infrastructure, difficult access and delayed rescue potential D High Specialist preparation, lighting and protective equipment may be essential D High Barefoot use may impair movement and emergency escape D High Reduced clothing may compromise warmth and physical protection D High Remoteness increases the consequences of injury or exposure D High Remoteness does not make nudity safe, lawful or private
Class A Very limited interaction with natural underground environmental factors.
Class B Limited interaction with selected natural underground environmental factors.
Class C Moderate interaction across several natural underground environmental factors.
Class D High interaction with a broad range of natural underground environmental factors.
Class E Exceptional interaction requiring a clearly richer combination of geology, water, acoustics, natural surfaces and minimal artificial disturbance than a typical natural cave.

General classification method

Classifications reflect the combined opportunity for environmental interaction rather than guaranteed health outcomes. Depth, darkness, technical difficulty, danger, isolation, remoteness, clothing policy or reduced clothing are not scored as environmental benefits.

  • Natural rock and geological formations
  • Darkness and reduced artificial light
  • Stable underground temperature
  • Humidity and natural airflow
  • Natural acoustics and echo
  • Underground water
  • Natural surfaces
  • Movement through enclosed terrain
  • Geological and biological diversity
  • Built infrastructure
  • Artificial lighting
  • Direct surface interaction
  • Clothing and footwear as modifiers

Every clothing and footwear column receives an environmental interaction grade. A change of clothing or footwear increases the grade only where it meaningfully changes direct environmental interaction.

Reduced clothing, nudity or footwear removal does not automatically increase the grade. Where these conditions add little meaningful interaction, the grade remains unchanged.

Red warnings identify circumstances in which footwear removal, reduced clothing or nudity may be unsafe, restricted, unlawful or incompatible with technical cave requirements. The warning does not remove or replace the environmental grade.

Important distinction: The class shown in each cell represents the environmental interaction opportunity available under that clothing or footwear condition. A red warning identifies legal, operational, environmental or safety limitations. An environmental grade does not provide permission to enter closed caves, leave managed routes, disturb formations, trespass, ignore operator directions or disregard rescue and emergency guidance.
Important: Class E is not awarded because a cave is deeper, darker, more remote, more dangerous or technically difficult. It is reserved for an exceptionally rich combination of natural geology, water, acoustics, natural surfaces, geological or biological diversity and minimal artificial disturbance. A high environmental class does not mean that deeper entry, reduced lighting, barefoot use or reduced clothing is preferable. Cave environments may require helmets, multiple light sources, protective clothing, sturdy footwear, thermal layers, route knowledge, trained companions and emergency planning.

How to read the Cave & Underground Health & Wellbeing Matrix™

Select the cave or underground environment that most closely reflects the actual setting, then select the clothing and footwear condition that represents the real interaction.

The resulting class represents the relative opportunity for environmental interaction under the stated assumptions. It is not a cave-difficulty rating, medical recommendation, air-quality assessment or instruction to enter unmanaged underground areas.

The row defines the environment

Natural rock, geological formations, darkness, humidity, water, acoustics, route complexity, infrastructure and artificial influence determine the primary underground setting.

The column modifies interaction

Clothing and footwear alter thermal protection, skin exposure and direct contact with rock and ground without changing the cave’s underlying environmental richness.

Warnings remain separate

A red warning identifies legal, operational or safety limitations while the A–E class continues to represent environmental interaction opportunity.

Environmental richness and technical cave safety are different

A cave may receive a Class E environmental rating while also requiring helmets, multiple light sources, sturdy footwear, protective clothing, trained companions and specialist rescue planning. High interaction does not mean that deeper or more difficult entry is advisable.

What may change a cave or underground classification?

A classification changes only where meaningful differences exist in environmental interaction. Depth, darkness, danger or route difficulty alone does not determine the class.

Natural geology Rock walls, mineral formations, lava structures, sediment and natural chambers increase geological interaction compared with artificial underground spaces.
Artificial infrastructure Paving, railings, concrete, mechanical ventilation and extensive lighting may reduce direct interaction with the natural underground environment.
Darkness Reduced natural and artificial light contributes to the underground sensory environment but also increases navigation and safety requirements.
Humidity and airflow Underground moisture, still air and natural ventilation influence thermal and sensory conditions.
Natural acoustics Echo, water, dripping, airflow and reduced surface noise contribute to the distinctive acoustic character of caves.
Underground water Streams, pools, seepage and waterfalls increase environmental complexity while also introducing flooding, cold and contamination risks.
Natural surfaces Rock, gravel, clay, sand and mineral deposits provide more direct interaction than built floors and platforms.
Movement opportunity Walking and observation may broaden interaction where routes remain stable, known and within personal capability.
Footwear Removing footwear may increase direct surface contact, but sharp rock, slippery ground, contamination and evacuation needs may make protection essential.
Clothing Reduced clothing increases skin exposure but may reduce protection from cold, abrasion, water, mud and rough surfaces.

A gradual cave and underground progression

The following progression illustrates one possible pathway towards greater underground interaction. It remains optional and should always reflect access, lighting, air quality, flooding, route complexity, physical capability and technical competence.

1

Begin in an artificial underground visitor space

Become familiar with enclosure, reduced daylight and underground acoustics in a fully managed setting.

2

Visit a commercial show cave

Experience natural formations, humidity and cave temperature while remaining on managed paths with lighting and supervision.

3

Spend time at a natural cave entrance

Observe the transition between daylight, vegetation, rock, airflow and partial enclosure without entering technical terrain.

4

Explore a shallow natural cave

Enter only where lawful, stable and suitable, using reliable lighting and remaining close to the exit.

5

Visit a lava tube or large chamber

Experience greater geological variety and underground acoustics on a known, appropriate route.

6

Observe underground geology

Spend time observing formations, mineral surfaces, water and cave structure without touching or damaging fragile features.

7

Increase duration gradually

Extend underground time only where temperature, lighting, orientation, physical comfort and exit access remain reliable.

8

Adapt footwear only where appropriate

Remove footwear only on stable, clean and non-technical surfaces with immediate access to protective footwear.

9

Adapt clothing only where protective needs allow

Choose clothing according to temperature, abrasion, water, mud, route conditions, operator rules and personal comfort.

10

Use lawful clothing-optional cave settings where available

Where lawfully established, clothing may be adjusted according to personal preference while maintaining footwear, lighting, thermal protection and immediate exit access where required.

Greater underground interaction does not require greater depth

Natural rock, acoustics, darkness and stable temperature may be experienced in accessible cave entrances and managed chambers. Moving deeper, entering water or using more technical routes does not automatically increase the environmental class and may sharply increase risk.

Safety and responsible cave use

Caves and underground environments may provide interaction with geology, natural surfaces, stable temperatures, humidity, darkness, water and distinctive acoustics. They may also contain hazards that are difficult to identify, avoid or escape once underground.

Reliable lighting

Each person entering beyond a naturally illuminated entrance should have a reliable primary light and appropriate backup lighting. A single shared light source may leave the entire group unable to exit safely.

Flooding

Rainfall outside a cave may cause rapid underground flooding, including when rain is not occurring directly above the entrance. Check forecasts, catchment conditions and local cave guidance before entry.

Air quality

Some underground spaces may contain low oxygen, carbon dioxide or other gases. Do not enter areas where air quality is uncertain, particularly confined, poorly ventilated or abandoned underground structures.

Rock stability

Loose rock, unstable ceilings, fractured walls and falling material may be present. Avoid touching, climbing on or moving beneath visibly unstable formations.

Footwear and head protection

Sharp rock, wet surfaces, mud, loose gravel and low ceilings may require sturdy footwear and a suitable helmet. Removing footwear may reduce traction and impair emergency movement.

Cold and wet conditions

Stable cave temperatures may still be substantially colder than surface conditions. Wet clothing, underground water and inactivity may increase heat loss.

Navigation

Passages may appear similar and apparently simple routes may become confusing in darkness. Use known routes, remain with the group and avoid relying solely on memory.

Restricted passages

Narrow spaces may create entrapment, panic, breathing difficulty or inability to turn around. Do not enter passages beyond personal capability or without appropriate training.

Underground water

Pools and streams may be deep, cold, contaminated or fast-moving. Water levels and currents may change rapidly, and apparently shallow water may conceal holes or obstacles.

Communication and rescue delay

Ordinary mobile communication may not function underground. Rescue can be technically difficult and delayed, making group planning and surface contact important.

Wildlife and biological material

Avoid disturbing bats, nesting animals and specialised cave species. Animal waste, fungi and contaminated surfaces may present health risks.

Conservation and cultural protection

Do not touch formations, remove minerals, damage cave surfaces or enter restricted cultural, archaeological or ecological areas.

A high environmental class is not a cave-entry recommendation

Class D describes high interaction with natural underground environmental factors. It does not mean that deeper entry, technical passages, water, barefoot use, reduced clothing or unsupported exploration is safe, beneficial or appropriate.

Evidence-first interpretation

The Cave & Underground Health & Wellbeing Matrix™ is a comparative environmental framework. It considers underground geology, natural surfaces, temperature, humidity, darkness, acoustics and water without claiming that one class guarantees a specific health outcome.

Geological interaction

Natural rock, mineral formations, sediment, volcanic structures and underground water contribute to the environmental character of caves.

Natural acoustics

Echo, dripping water, airflow and reduced surface noise may create distinctive acoustic conditions. Individual responses to enclosed sound vary.

Stable temperature

Many caves have comparatively stable temperatures, but stability should not be confused with comfort or protection from cold stress.

Darkness and reduced visual input

Reduced light alters sensory perception and orientation. It may also increase anxiety, disorientation and dependence on artificial lighting.

Humidity and air movement

Humidity and ventilation contribute to underground thermal and sensory conditions. Actual air quality must be assessed separately.

Movement and uneven terrain

Walking through natural passages may contribute to physical activity while also increasing fatigue, slipping and injury risk.

Natural surface contact

Rock, gravel, clay and other natural surfaces may increase sensory interaction, but direct skin or barefoot contact should not be assumed to provide a health benefit.

Clothing and protection

Clothing and footwear modify thermal protection, traction and resistance to abrasion. In most caves, reducing protection does not meaningfully increase the environmental class.

Classification is comparative, not clinical or technical

The A–E scale compares opportunities for interaction with natural underground environmental factors. It is not a medical prescription, cave grade, air-quality test, flooding assessment, geological inspection or substitute for professional cave-safety guidance.

Clause de non-responsabilité

The Cave & Underground Health & Wellbeing Matrix™ is provided for general educational, research and policy-development purposes.

It does not provide medical, psychological, legal, geological, structural, air-quality, flooding, caving, navigation, rescue or emergency advice. It does not guarantee health improvement and should not be used to determine whether a cave, underground route or activity is safe for a particular person.

The matrix assumes lawful access and environmental conditions suitable for limited human activity unless otherwise stated. Actual rock stability, air quality, water level, temperature, humidity, route condition, lighting, communication and emergency access may vary substantially.

Individual experience, mobility, health status, physical capacity, claustrophobia, clothing, footwear and response to darkness or confinement will vary.

Users remain responsible for checking weather, catchment conditions, cave closures, access permission, operator requirements, protected-area rules, cultural restrictions and advice from relevant authorities or experienced cave professionals.

Anyone with respiratory, cardiovascular, mobility, balance, neurological, temperature-regulation, anxiety or other concerns should seek appropriate professional advice before undertaking unfamiliar underground activity.

Supporting evidence and references

The final published reference list should support the principal variables used within this matrix and clearly distinguish established evidence, emerging evidence and NRE’s comparative classification methodology.

  1. Public-health and environmental-health research concerning nature exposure, outdoor or nature-based activity and selected measures of wellbeing.
  2. Research concerning human responses to enclosed natural environments, darkness, sensory change and underground spaces.
  3. Geological and speleological sources concerning cave formation, natural surfaces, mineral structures, underground water and cave environments.
  4. Evidence concerning acoustics, environmental sound, echo and human responses to distinctive sound environments.
  5. Research concerning stable underground temperatures, humidity, ventilation and thermal comfort.
  6. Evidence concerning walking, climbing, crawling, uneven terrain, fatigue, balance and cave-related injury.
  7. Public-health and occupational guidance concerning low oxygen, carbon dioxide, toxic gases and confined-space hazards.
  8. Government, rescue and caving-organisation guidance concerning cave lighting, navigation, flooding, group planning and emergency response.
  9. Conservation guidance concerning bats, cave fauna, archaeological material, mineral formations and fragile underground environments.
  10. Relevant legal and policy sources concerning cave access, protected areas, private property, public nudity and lawfully managed clothing-optional settings.

Related NRE resources

This matrix forms part of the wider NRE Health & Wellbeing Matrix™ Library and should be interpreted alongside related environmental, educational and policy resources.

11 Levels of Health Restoration™

Explore NRE’s progressive framework for increasing interaction with outdoor and natural environments according to individual comfort and capability.

View the 11 Levels framework

NRE Guides Hub

Access verified NRE guides covering health, research, policy, standards and institutional programmes.

Visit the NRE Guides Hub

NRE surveys

Participate in ongoing NRE Health Institute research concerning health, wellbeing, stigma and non-sexual nudity.

View current NRE surveys

Version history

Version records support transparency, methodological review and future refinement of the Cave & Underground Health & Wellbeing Matrix™.

Version Status Summary
Version 1.0 Initial general edition Established cave and underground environment categories, the A–E interaction scale, clothing and footwear modifiers, underground safety boundaries and evidence-first interpretation.
Version 1.1 Grading review Corrected over-grading of large, deep, technical and remote caves. Technical difficulty, danger, depth and remoteness are no longer treated as environmental benefits.
Methodology standard Current Reduced clothing or footwear changes the grade only where it meaningfully changes environmental interaction. Clothing policy, technical difficulty and risk do not determine environmental class.
Class E threshold Current Class E is reserved for an exceptionally rich combination of natural geology, water, acoustics, natural surfaces, geological or biological diversity and minimal artificial disturbance that clearly exceeds a typical natural cave.
Future review Planned Future editions may refine variables, cave categories, supporting evidence, jurisdictional guidance and specialist applications following research review and stakeholder feedback.

Cave interaction should remain informed, lawful and properly protected

Caves can provide distinctive opportunities for observing geology, underground water, natural acoustics, darkness and stable environmental conditions.

The strongest environmental classification is not necessarily the deepest, most remote or most technically difficult setting. Clothing, footwear, route, lighting, duration and activity should remain guided by protection, competence, conditions and reliable exit access.

Environmental interaction may be encouraged. Underground risk should never be treated as environmental value.