Near the Norwegian border somewhere in the depths of the Arctic Circle in Russia lies the deepest hole ever drilled, once called "the entrance to hell." The hole was a man-made creation through ...

Without boots or machinery on the ground, it's hard to tell what is a cave entrance, and what is simply a hole. And it's impossible to know how far any of the caves extend beneath the surface.

This hole might lead to underground caves. Scientists think these caves could hold alien life. The US Geological Survey has mapped many potential cave entrances. NASA plans to explore these pits ...