A punishing path and 90 percent humidity test even the fittest hikers on a grueling trek to two of the world's largest known caves. https://on.natgeo.com/2WQoApu

A punishing path and 90 percent humidity test even the fittest hikers on a grueling trek to two of the world's largest known caves. https://on.natgeo.com/2WQoApu 

https://twitter.com/NatGeoTravel/status/1128737016215166981