The Earth's bimodal topography reflects the 2 distinct types of crust found on our planet. Global topography is what the surface looks like, an image of the planet showing both land and water.
Density and thickness contrast between granite and gabbro (continent vs. ocean floor) both promote relatively high continents and relatively low ocean floors. Density, therefore, is a first order property that explains Earth's characteristic bimodal elevation distribution. Learn more at Globalchange.umich.edu.
Below sea level oceanic crust is thin (˜7 km on average), consisting of relatively dense rock. In contrast, the high-standing crust is thick (˜40 km on average), and is composed of highly varied lithologies that yield an average intermediate bulk composition and contains the oldest rocks and minerals yet observed on Earth.
The “hypsometric graph” is a statistical summary of Earth's surface elevations: 30% of Earth's surface is above mean sea level (mean land elevation around 820 m); 70% of Earth's surface is below mean sea level (mean depth around -3700m.) This bimodal distribution of surface elevations reflects the two variations in crust composition.
Mars Express, the first planetary explorer for the European Space Agency, has been taking images of the Red Planet. See the topography of Mars in full color at CNET.