The air temperature drops.

The sound of wind, insects, birds — the ambient sounds of the outdoor environment — fades and is replaced by a different kind of sound: the faint dripping of water, the shift of rock under a boot, the sound of the caver’s own breathing amplified by the narrow walls.

The smell of the cave is distinct and specific: cold clay, mineral-laden water, the slightly organic smell of cave sediments.

To go deeper is to move into increasing unfamiliarity.