Volcanoes have long helped shape the Earth. But what is less well known is that there are volcanoes on other planets and moons that are even more extraordinary than those on our own home planet. Horizon follows an international team of volcanologists in Iceland as they draw fascinating parallels between the volcanoes on Earth and those elsewhere in the solar system. Through the team's research, we discover that the largest volcano in the solar system - Olympus Mons on Mars - has been formed in a similar way to those of Iceland, how a small moon of Jupiter - Io - has the most violent eruptions anywhere, and that a moon of Saturn called Enceladus erupts icy geysers from a hidden ocean. Computer graphics combined with original Nasa material reveal the spectacular sights of these amazing volcanoes. Along the way, we learn that volcanoes are not just a destructive force but have been essential to the formation of atmospheres and even life. And through these volcanoes of the solar system, scientists have discovered far more about our own planet - what it was like when Earth first formed, and even what will happen to it in the future.