Seafloor exposed.

Until recently, scientists knew more about the surface of Venus than about the underwater surface of our own planet. But now researchers have patched together Navy satellite photos to map the ocean floor more accurately than ever before.

The map (shown above) shows the rises and drops in the ocean floor, which scientists can use to figure out where mountains, valleys, and other seafloor features lie.

"This is the first map of the entire ocean floor," which covers 70 percent of the globe, says Charles De Met, a geophysicist at the University of Wisconsin.

The map looks like someone drained all the water from the ocean, adds Jian Lin, a geophysicist at Woods Hole Oceanographic Institution. Now scientists can see rows of squiggly lines--Earth's fracture zones (cracks on the ocean floor). These cracks, like scars from a wound, indicate the directions in which tectonic plates (slabs of Earth's cruso have shifted over the past 200 million years, De Met says. Scientists will study these fracture zones to learn more about continental drift--how chunks of Earth's crust split apart to form the continents.

The map also reveals several surprises, including twice the previously known number of seamounts, or undersea volcanoes, says Lin. Scientists can now pinpoint these volcanoes to measure temperatures in the mantle--the layer of hot rock beneath Earth's crust. "In places like Hawaii," Lin explains, "the mantle is so hot that the rocks melt and erupt from the ocean floor to form undersea volcanoes.

"We still have a lot to learn about the geology of the ocean floor," Lin says, "This map will guide us toward figuring out the forces that shape our planet."