If space is the final frontier, then where does that leave the depths of our own planet? As it turns out, both environments are risky and dangerous, and the European Space Agency is using underground caves as training for astronauts before they go up together. The focus is really teamwork and learning to trust each other and the process. It’s tough to find an environment that mimics that of outer space, but these underground caves seem to be foreign enough to make it work. National Geographic reported on the training program where astronauts explore deep beneath the surface.