The perception of sound underground is a very unusual experience compared to overground - the environment is unfamiliar, awkward and potentially hostile, but it is also an extremely useful laboratory for the study of all manner of physical and auditory phenomena. The conductivity of rock is excellent compared to air, and thus sounds can be transmitted long distances and with great clarity through the medium - and our proximity to this medium when we are underground ensures that we also can hear (or more accurately, feel) the sounds transmitted, in a way not possible on the surface. Cavers exploring within the Peak-Speedwell cave system in Castleton may be ‘fortunate’ to encounter a blast at the nearby Hope Valley cement quarry. Entirely unpredictable (unless you know the shift-times), these incomprehensible booms suddenly interrupt a caving session, with their hollow strength felt throughout the body. As we are effectively within the rock, the sound passes through us as a giant wave of energy - deep, solid and yet transient. The reverberation space of the huge tunnels enhances this effect, creating an enormous hollow drumbeat of geological intensity.

The range of sounds that can be produced (and recorded) underground is enormous - thanks to the endless variety of cavern size, amount of fill present, length of open passage beyond, the extent of draughting connections to surface, the amount and velocity of water moving through a cave, or simply the still waters standing in it - all these factors can contribute to an almost infinite set of ‘recording studios’. Coupled with the variance of the materials present that may be used to produce sounds (rock, water, sand, clay, wet mud, etc.), the permutations available for capturing interesting sounds are virtually limitless. Sounds produced from rock and water alone can range from dull, but very powerful booms, more felt than heard, through deep and rich percussive tones suggesting drums, to highly chromatic metallic bell/chime sounds and impossibly high-pitched clicks, clatters and splats, moving into the ultrasonic registers, with notes perceived only by bats and birds. Many of these tones have a recognisable pitch, implying that these sounds could, theoretically, be used to produce organised ‘music’.

The objects capable of producing the purest, most melodic tones are without doubt speleothems - the vertically-oriented calcite deposits more commonly known as stalagmites and stalactites, but which also can create ‘curtains’ - sheets of translucent flowstone that can produce a variety of tones from one object. When struck, these can emit a beautiful sound, soft and yet very pure. Conservation considerations mean that only robust and solid formations should ever be struck, and only then with soft-tipped objects. Nevertheless, it is probably the closest to a natural ‘instrument’ that nature is capable of producing, and the Great Stalacpipe Organ, a lithophone in Luray Caverns, Virginia, USA, specifically utilises these qualities, using a vast array of levers, cables and hammers to ‘play’ the cave, via a conventional keyboard; this is perhaps the most literal use of a cave to make not only sound, but beautiful and quite unique music.

Within a cave environment, as the dominant sense of vision becomes less useful, one’s remaining senses are heightened to levels unnoticed on the surface, and more primitive, normally subdued instincts may become more prevalent. Touch and smell particularly become more sensitive, as they must, in order to physically move through the difficult environment and to navigate away from or toward the connection to surface. Temperature-sensing and draught detection also assume a higher role. But it is the auditory senses that perhaps undergo the greatest transformation - removed from the surface atmosphere (and excepting large open entrances and occasional dig breakthroughs) there is no strong wind underground to create background noise. The endless traffic sounds of the city and its street life are removed, and even in the quieter countryside, birdsong and other animal noises are all gone, leaving the slow, silent emptiness of the cave space as a blank canvas to project one’s auditory imagination onto.

Traversing through a cave, with its myriad forms and endless permutations of `empty space’ and `object’ can produce auditory experiences in a constant sense of flux, as the acoustic properties (or ‘signatures’) of each individual space slowly morph from one to another; ranging from huge echoing caverns, to the dull anechoic softness of a small, spherical sediment-filled chamber, and the especially unusual ‘drainpipe’ sounds only experienced in tight tubes and flat-out crawls. All these spaces produce a vast amount of different auditory stimuli that we become able to ‘read’, with practice, using our most primitively-encoded senses, developed millions of years in the past through the physical evolution and learned experiences our common ancestors. Animals, and then humans lived within a cave environment where possible, as their ready-made usefulness as shelters was obvious, and the ability to read an acoustic space, especially with little or no light, would become a primary function of early human life, especially when they might be sharing these shelters with predatory animals.

If our ancestors passed through cave passages regularly, often wading through static pools of water or clambering over loose rocks, they would learn the acoustic signal of each space, building up a ‘sound map’ in their minds to help them pass through safely. Each chamber would resonate in different ways and with their conversation to help reflect the sound, even in very dim (or no) light, it would be possible, as with bats, to ‘navigate’ almost using hearing alone. It is also possible that as early humans heard the various tones that the elemental materials themselves create - the soft, thumping ‘gloops’ of water hitting a cave wall in a restricted space, the high-pitched, clattering, crockery-like sounds of limestone slabs as they stepped over them - these sounds could coalesce in the human mind to produce the idea of ‘music’. That is, sympathetically-arranged sounds that impart a meaning, as opposed to an accidental noise or even a warning signal. Melodic construction could encode information, and thus allow folk-memories to be transmitted down generations. Drums and other percussion instruments have always been used to enhance ritual practises, and in an underground environment these would have been even more dramatic, with reverberation and resonance adding further drama. It is easy to imagine that the original inspiration for these rhythmic creations might have evolved from the natural sounds produced by humans or heard naturally within caves.

Of course, the cave itself may produce sound - a fast-flowing stream will make plenty of noise on the surface, but underground, with no place for the sound to leave, it can be deafening, with the resultant cacophony of water splashing, gurgling, folding and popping creating an auditory environment which can be either terrifying or tranquil, depending on perspective and context. The ‘white noise’ so beloved of some meditative practices is easily achieved in these circumstances underground, and the removal of most general stimulus can be conducive to slipping ‘outside the moment’, into a timeless and boundless mental space. Alternatively, when listening in a ‘quiet’ cave, with only constant drips of water for company, the rhythmic aspects of these sounds, and our contemplation of these rhythms could also produce ‘transcendent’ experiences. The differing frequencies of rhythm caused by water drips and the variety of their tonal qualities on hitting different surfaces is always fascinating, particularly when the drips begin to cut a hole in the surface, making a tiny ‘water drum’. Each drip pattern has its own timing cycle, which may briefly coincide and then diverge again from other drips, all running in their own unique cycle - it may be many hours before the entire ‘suite’ of drips exactly repeats itself, if ever. Varying rates of rainfall and the subsequent random transmission of water through rock ensures that there is virtually no ‘constant’ here other than the process itself.

As the most underground-accomplished tradesmen, miners have always been adept at ‘reading’ underground spaces - especially in ancient times or in less technologically-developed places, with a heavy reliance on their own senses, - their auditory faculties would be extremely highly-developed, relying on hearing far more than sight when working in dim candlelight. Each strike of a tool against hard rock would inform a miner as to the nature of what lay beyond - a hard, piercing ring would indicate solid immovable rock, a less bright sound would be produced by vein material or other mineral, whilst a dull, hollow thud would be made by clay, or loose or fractured rock, or even a hidden void about to be uncovered. Sounds of running water would be particularly useful, signalling imminent danger from flooding, or conversely usefulness due to drainage potential - and possibly an easier way on to further mineral finds. The dull booms of explosives would resonate through the mine, each blast releasing tons of rock fragments and dust, themselves also creating additional sounds, and by partially filling the tunnels, changing the acoustical environment again, albeit temporarily.

The use of sound to determine risk was probably used to best advantage when dealing with the deceptively pretty but very dangerous ‘slickensides’ - smooth and flat surfaces of rock on the opposing cheeks of a vertical and narrow fissure, revealed by the removal of a mineral vein. These surfaces were polished by the original earthquake that made the fault, sliding the rocks at extreme pressure and heat past each other until their surfaces were rendered perfectly smooth. As the minerals slowly filled the opened gap, created via the upwelling of highly saturated thermal waters whilst buried deep underground, enormous tensions and pressures could then be created by much later earthquakes, with the fault slightly closing, or slipping again. Removal of the vein by the miners could thus release the stored tension, often catastrophically so, and therefore they would often employ a long iron rod to test the walls, the other end of which they would strike from a safe distance with a hammer, listening for any unusual sounds - creaks or groans which might betray an imminent failure of the face. Miners were killed by not following this precaution or ignoring the information their senses were relaying to them.

In recent times, our day-to-day requirements for excellent hearing have diminished, and auditory spaces are often now less defined and hold less value in our everyday lives. The delivery of musical and visual media, and the background cacophony of modern life can be so intense that opportunities for ‘careful hearing’ have been dramatically reduced. The echoing quiet of a church or other large and reasonably empty building can often trigger that ancient respect for silence and small noises, and thus contemplation is possible - in many cases this is the nearest humans may get to realising that their ‘normal’ acoustic environment has changed. The exaggerated and sometimes extremely strange effects on sounds produced in the endlessly varied and random cave environments can transform everyday reality into something completely different, and they link us directly back to our deepest pre-history, by re-awakening our ‘lost’ sensory abilities.