The Earth’s ocean floor, mapped with microwaves bounced off the ocean’s surface from spacecraft as well as with soundings from ships. Features on the seafloor with excess gravity attract the water enough to cause structure on the ocean’s surface that can be mapped, though a 2000-meter-high seamount produces a bump on the surface only 2 meters high. High gravity is shown in yellow, orange, and red; low gravity is shown in blue and violet. The feature running from north to south in the middle of the Atlantic Ocean is the mid-Atlantic ridge. It marks the boundary between plates that are moving apart. Continents drift at about the rate that fingernails grow.
Tides