liquid water flowing beneath their surfaces. The prime candidate for life beyond Earth is Jupiter's moon Europa. During the late 1990s NASA's Galileo spacecraft spent several years in orbit around Jupiter. It discovered that Europa has a vast ocean beneath a thick crust of ice. The ocean completely surrounds the moon, and may be more than 100km deep. This means that there may be more water on Europa than in all of Earth's oceans put together. I journeyed 2km down to the ocean floor in the Sea of Cortez, Mexico, to see how life can exist on Earth in conditions thought to be similar to those on Europa. At these depths the pressure is more than 200 times that on the surface, and the US submarine Alvin is one of the few vessels that can survive it without being crushed. Yet we found living creatures. We also filmed in a glacial cave carved out of solid ice in Iceland. There are microbes living deep in the ice that have been frozen for thousands of years. If life can exist without sunlight deep beneath the oceans of Earth and in the heart of glaciers, then who knows what might be swimming around on Europa? It is now also thought that at least two more of Jupiter's large moons, Ganymede and Callisto, may also have salt water oceans beneath their surfaces. Perhaps there is life there too. Closer to home, the evidence that life could exist on Mars is growing. NASA's robot