Larisa Pozdnyakova’s words, in her thick Russian accent, float to me from within the cave ... and the map looks like a multicolored schematic of the human circulatory system.

On tours of Mammoth Cave, guests can view the various canyon, large canyon composite, keyhole, tube ... Nahum Ward's 1815 ...

from which surface water makes its way underground, eroding the limestone into a honeycomb of caverns. Mammoth, the world's longest known cave system, a UNESCO World Heritage site and the core ...

And some pits on the surface could be skylight-like entrances to the lava tube system below ... lead to caves in 2012. Launched in 2009, the LRO was conceived to create a 3D map of the moon ...