The striking photograph above appears in higher resolution in "You and the Universe" on page 85.  At its center is the largest canyon in the solar system, Valles Marineris. Ten times as long and four times as deep as the Grand Canyon, it goes one-fifth of the way around Mars. Most scientists think Valles Marineris is a large crack in the Martian crust that formed as the planet cooled, later to be widened by erosion. Near its eastern flanks are channels that may have been formed by water. Similar outflow channels tens of kilometers wide, hundreds long, and up to a kilometer deep suggest that an immense reservoir of water was present during much of Mars’ history. Based on conservative estimates, it’s been calculated that at the time they formed, Mars possessed the equivalent of a global ocean half a mile deep, which today may be buried as groundwater beneath the surface. Mars’ axial tilt has varied between 0° and 60° in the last 10 million years, which may have triggered huge climactic changes as these vast subsurface caches were vaporized and driven into the atmosphere. At the left are three very large volcanoes (the third is in shadow, center left) sitting on top of a big bulge in Mars called the Tharsis Ridge.