There's a fascinating tidbit in this Google blog post about the non-discovery of Atlantis in Google Earth. It concerns how the depth of the ocean floor is measured.
Now you're probably wondering where the rest of the depth data comes from if there are such big gaps from echosounding. We do our best to predict what the sea floor looks like based on what we can measure much more easily: the water surface. Above large underwater mountains (seamounts), the surface of the ocean is actually higher than in surrounding areas. These seamounts actually increase gravity in the area, which attracts more water and causes sea level to be slightly higher. The changes in water height are measurable using radar on satellites.
Wow! (via ben fry)