What exactly is Valles Marineris? It’s a scar across the surface of Mars over 2,000 miles long, which some have called ‘the greatest abyss in the solar system’. First spotted by probes in the early 1970s, it has intrigued scientists ever since. Many theories have been put forward to explain its origin: water erosion, tectonic splits, a rift valley. Dan James’ fascination with Valles prompted him to investigate them in his first popular science book and to come up with some enthralling answers.