What is the deepest cave system known in the world?

Veryovkina in the Caucasus, at 2,212 metres - and the record has changed hands repeatedly between it and neighbouring Krubera in the same limestone massif, as expeditions push a few metres further. Both are in Abkhazia, in a mountain riddled with the deepest known caves on Earth.

Reaching the bottom takes a team weeks, hauling equipment down vertical shafts, camping underground, and squeezing through passages that flood. In 2018 a team at the bottom of Veryovkina was caught by a flood pulse that raised the water by over a hundred metres in hours; they abandoned everything and climbed out. Cave diving in flooded sections is separately among the most lethal activities anyone does for pleasure.

Depth records are also a statement about exploration rather than geology. There is no reason to think these are the deepest caves that exist - only the deepest anyone has got to the bottom of, in a discipline where progress is made in metres per expedition. It is one of the last fields where a well-organised amateur group can still discover somewhere no human being has been.