Caving

The oldest known cave markings — red hand prints and animal sketches in Spanish caves — are over 65,000 years old and were made by Neanderthals, not modern humans. As a space for symbolism, caves are older than our species in Europe.

The Veryovkina Cave in Georgia is, at 2,212 meters deep, the deepest known cave in the world — the full descent takes 27 days, with underground camps and dive passages through siphon-like water constrictions.

Three independent light sources are the minimum. Cave darkness is absolute — your eyes don't adjust, because there's no light to adjust to. Anyone who goes in without a backup lamp is risking a tragedy.