Caves remain one of the last unexplored frontiers on Earth. Yet, unlike space or the deep ocean, cave exploration is available to the common man or woman; with training and minimal equipment (a helmet and three sources of light), it is possible to walk where no human has walked before. Nonetheless, the dangers of cave exploration are very real, and not every individual looks forward to the idea of 100-m (330-ft) pits, or crawls so tight that one must exhale to fit. Therefore, the exploration of these passages, as both a human and scientific endeavor, is well behind that of the other terrestrial areas of our planet. As scientists begin to examine this environment, they are making discoveries that are changing our ideas on how caves form and the role that microbial species play in this unique environment.