Jupiter, the fifth planet from the Sun and the largest in the solar system, is orbited by 69 moons, including lo, Europa, Ganymede, and Callisto. Europa, which is of the same size as our Moon, shows crisscrossing cracks in the surface that change on time scales of weeks or months. To expert geologists and planetary scientists, this behavior implies that Europa has a surface made almost entirely of water ice, like a giant Antarctic ice sheet enclosing an entire world. And the changing appearance of the cracks in this icy surface leads to a startling conclusion: the ice apparently floats on a worldwide ocean. Only by assuming the existence of liquid beneath the icy surface can scientists satisfactorily explain what they have seen, thanks to the stunning successes of the Voyager and Galileo spacecraft, which were sent to study Jupiter and its moons. Since we observe changes on the surface all around Europa, we may conclude that a worldwide ocean of liquid must underlie that surface.