Now imagine them down there, Frank and Cookie, mud smeared and grinning, on the verge of joining two cave systems into a single, immense whole. Not your thing? Well, for cavers, it's the thing. And it's rare that such superlative connections are made. In the often obscure world of underground exploration, which is governed by international bodies with names such as the "Longest, Largest, and Deepest Committee," such a feat would be a very big deal.

Elsewhere far below the Earth's surface, in the entrails of Racer Cave, another team was slithering into place. They too carried hammers and a drill, and soon the two teams would begin banging on the cave walls and drilling into the rock, listening for each other, hoping noise would lead them to a connection and a spot in the record books.