Rainwater is very slightly acidic, and that is enough
Rain picks up carbon dioxide on the way down and more of it from the soil, which turns it into a weak acid — far too weak to harm you, strong enough to dissolve LIMESTONE. Water works into a crack, widens it by a hair, more water follows the easier route, and over tens of thousands of years a crack becomes a TUNNEL and a tunnel becomes a CAVERN.
Inside, the process runs backwards. Water arriving through the roof loses its carbon dioxide to the air and can no longer hold the dissolved stone, so it leaves a speck behind at every DRIP. A STALACTITE grows down at something like a centimetre a century; a STALAGMITE builds up underneath from what the drip carries to the floor. Meet in the middle and they make a PILLAR. The memory trick that works: stalactites hold tight to the ceiling.
A cave is the same temperature all year
Go far enough in and the seasons stop. Below the depth that surface warmth reaches, a cave settles at roughly the local average annual temperature and stays there — COOL in August, distinctly mild in January, DAMP always. In southern England that is around eleven degrees, year in and year out.
People exploited this long before refrigeration. Caves and cellars were the larder, the ice store and the place you aged cheese and wine, because a steady temperature does the work that a thermostat does now. Roquefort is still legally required to be ripened in the caves at Combalou, where the cool DAMP air and the moulds in it do something a factory has never managed to copy.
Animals in the dark stop bothering with eyes
The Mexican tetra is an ordinary silver river fish with ordinary eyes. The same species also lives in caves in Mexico, and those populations are pale and blind — and the striking part is that this has happened independently in cave after cave, each one losing its eyes separately.
Stranger still, the blind ones start to build eyes as embryos and then stop; the tissue degenerates before hatching. The instructions are still in there, switched off. In permanent DARK an eye is expensive to grow and useless, so anything that spends the energy elsewhere does better. Cave animals across the world, from the olm in Slovenia to blind cave shrimp, keep arriving at the same pale, eyeless design.
What is down there, and how far down
The grid has the architecture — the CAVERN, CHAMBER and TUNNEL, the LIMESTONE ROCK they are cut from, a fallen BOULDER, the odd CRYSTAL, and the STALACTITE, STALAGMITE and PILLAR that the DRIP builds. Then the conditions: DEEP, DARK, COOL, DAMP, HIDDEN, and an ECHO for anyone who shouts.
The deepest cave anybody has reached is Veryovkina in Georgia, over two thousand two hundred metres down — deep enough that getting to the bottom and back is a multi-week expedition with camps underground. A caver's TORCH is the only sun they get. The BAT has the easier arrangement: it uses the entrance chamber, sleeps through the day and leaves at dusk to EXPLORE somewhere with insects in it.