The difference is that this ocean is deep within the Earth's mantle, some 400 miles below the surface. To figure out what was going on down there, the researchers studied seismic waves generated by more than 500 earthquakes. These waves can continue vibrating for days as the shockwaves pass through the Earth's mantle and core, reflecting back and forth. By measuring their speed, we can learn about the material they are passing through. Dr. Steve Jacobsen at Northwestern says that scientists have been able to study two diamonds from this area deep in the Earth's mantle, that were expelled during volcanic eruptions.