They accumulated the knowledge through generations of underground work.

And the courage — the willingness to go into absolute darkness two miles, three miles, farther, to do the work and then find the way back — was simply part of what the operation required, and part of what the communities who conducted it trained and selected for.

Patty Jo Watson called them the world’s greatest cave explorers, and the characterization stands.

No other ancient population on Earth is documented to have penetrated so deeply into a cave system, so far from sunlight, using only handheld fire, over so long a period of time.