Cavers explore upside-down Everests

Forget mountain climbing or arctic exploration. Miles and miles underground, a more extreme adventuring is taking place — caving.

On expeditions that can last months and require weeks below the surface of the earth, cavers are constantly trying to go deeper and deeper, braving strange diseases, risky scuba diving in underground lakes and other dangerous unknowns.

Journalist Burkhard Bilger's latest New Yorker piece follows an expedition of the world's most advanced cavers into Cheve — possibly the deepest cave in the world, located in Mexico. Bilger describes the effort as "a kind of Everest expedition turned upside down."

And that underground world contains different kinds of risk:

Caves are like living organisms, James Tabor wrote in "Blind Descent" ... . They have bloodstreams and respiratory systems, infections and infestations. They take in organic matter and digest it, flushing it slowly through their systems.

Bilger joins The Daily Circuit to discuss the sport and the cavers who practice it.