In the past two decades, NASA spacecraft have identified potentially habitable environments throughout the solar system and beyond. Spacecraft on Mars have found evidence that lakes and streams once covered the planet, protected by a long-gone thick atmosphere. At Saturn’s moon Enceladus, the Cassini spacecraft sniffed plumes of water jetting out of Enceladus’s icy shell—detecting chemistry akin to that occurring in certain places on Earth’s seafloor, where seawater chemically reacts with rock (and where living creatures thrive). The upcoming Europa Clipper mission could do the same at Jupiter’s moon Europa, where scientists have spotted more plumes. Even beyond the solar system, some of the thousands of planets now known to orbit other stars may harbor surface oceans.