If there's life on Mars, it would be one of the most important discoveries of all time. It would mean life on Earth was not some special unique event - it would mean there's likely to be life throughout the Universe. In the first of two special programmes on Mars, Horizon explores how the search for Martians is now hotting up, and why many scientists are becoming more and more convinced that life may have arisen on Mars, and that there may even be something living there now. Today, Mars is a frozen desert - the average temperature is -70ºC. But long ago Mars was very different. The first clues came from Mariner 9, which sent back fuzzy images of the surface of Mars, revealing volcanoes, canyons and meandering valleys that looked like ancient rivers. Rivers form from rain water run-off, rain comes from clouds, and clouds mean an atmosphere. If there had been rivers on Mars, it meant the planet had once been warm and wet, like the Earth - the perfect conditions for life to evolve.