Mars’ biggies best The Mail & Guardian

Scientists may have discovered the largest flood channels in the solar system on Mars, currently a cold desert planet. A system of gigantic ancient valleys – some as much as 200 kilometers wide – lies partly buried under a veneer of volcanic lava flows, ash fall and wind-blown dust in Mars’ western hemisphere. New observations made with the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft reveal northwestern slope valleys (NSVs) northwest of the huge martian volcano, Arsia Mons, and south of Amazonis Planitia, site of a postulated ocean.