 The team also visited volcanic caves on the Spanish island of Lanzarote, the closest thing we have to this moon cave. It's located in the Mare Tranquilitatis, a vast rocky plain visible to the naked eye from Earth. Helen Sharman, the first British astronaut to travel to space, told BBC News the cave has a lot of potential, but it's so deep that astronauts might need to abseil in and use jetpacks to get out.