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. For thirty years scientists like Mike Carr have been poring over more and more detailed images of Mars, trying to piece together the planet's history. When the latest probe, Mars Global Surveyor, was launched three years ago, everyone hoped its high resolution electronic cameras would settle the debate about the ancient rivers. Thousands of close up images were beamed back, revealing sections of the valleys in fantastic detail, but none of them were any use. Over the billions of years, the valleys had all been eroded and filled with sand. It was impossible to say how they had been formed. Then finally, the Mars Global Surveyor team noticed a tiny feature in one image, which convinced them that billions of years ago there were rivers and lakes on Mars. But did these warm, wet conditions last long enough for life to evolve? An unusual lake in Turkey has convinced Scottish geologist, Mike Russell, that Martian lakes were once teeming with primitive bacteria. So if life did evolve, is