Earth has some impressive volcanoes, but they’re nothing compared to Io, Jupiter’s chaotic little moon. Io is the most volcanically active place in the solar system, with lava fountains dozens of miles high and lakes of molten rock. Its constant eruptions are caused by Jupiter’s gravity, which stretches and squeezes Io like a stress ball, generating enough heat to melt its insides. If you ever visited, you wouldn’t find a calm, cratered moon—you’d find a landscape constantly reshaped by fire.