The worlds that surround our planet are all made of rock, but there the similarity ends. Some have a beating geological heart, others are frozen in time. Brian travels to the tallest mountain on earth, the volcano Mauna Kea on Hawaii, to show how something as basic as a planet's size can make the difference between life and death. Even on the summit of this volcano, Brian would stand in the shade of the tallest mountain in the solar system, an extinct volcano on Mars called Olympus Mons, which rises up 27 km. Yet the fifth wonder in the series isn't on a planet at all. It's on a tiny moon of Jupiter. The discoveries made on Io have been astonishing. This fragment of rock should be cold and dead, yet, with the volcanic landscape of eastern Ethiopia as a backdrop, Brian reveals why Io is home to extraordinary lakes of lava and giant volcanic plumes that erupt 500 km into the sky.