For millennia, caves have served as shelters for prehistoric humans. Caves have also intrigued scholars from early Chinese naturalists to Charles Darwin. A cave ecologist has been in and out of these subterranean ecosystems, examining the unique life forms -- and unique living conditions -- that exist in Earth's many caves. But what does that suggest about caves on other planetary bodies? In two connected studies, engineers, astrophysicists, astrobiologists and astronauts lay out the research that needs to be done to get us closer to answering the old-age question about life beyond Earth.