The difference between Mars' highest and lowest points is nearly 30 km (from the top of Olympus Mons at an altitude of 21.2 km to the bottom of the Hellas impact basin at an altitude of 8.2 km below the datum). In comparison, the difference between Earth's highest and lowest points (Mount Everest and the Mariana Trench) is only 19.7 km. Combined with the planets' different radii, this means Mars is nearly three times rougher than Earth.