The survey ship Challenge (pp.30-31) discovered that most of the ocean floor lies around 5 km (3 miles) below the level of the sea. this deep ocean floor is all made of relatively young rocks, none older than 200 million yeas. There was ocean floor in earlier times, but all of that old rock has disappeared, subducted into the Earth's interior at destructive plate boundaries (pp.36-37). New ocean floor is being made all the time by sear-floor spreading, which takes place at mountainous volcanic ridges. Here, magma rises from the mantle and fills the crack left as the ocean floor the pulls apart. The uppermost layer of the volcanic ocean floor is basalt lava. Beneath this is a layer with vertical structures, and below this a third layer made of coarse-textured gabbro. The Earth's magnetic field reverses its direction from time to time, and these reversals are recorded in the new ocean floor rocks as they crystallize.