Researchers aboard the ship Atlantis explored the sea floor. They used depth sounders to map the bottom surface of the Atlantic Ocean. These researchers discovered the Mid-Atlantic Ridge—mountains separated by canyons and valleys. A model called sea-floor spreading states that new crustal material is forming in the sea floor. It pushes apart the old sea floor on both sides of the ridges as it forms. Beneath, there is molten rock called magma. This flows up through the cracks, then cools and hardens. This process makes new rock material while pushing older rock material away along the sea floor. Other oceans also have mid-ocean ridges. Some of these ridges have huge cracks on top and other parts of the sea floor have deep valleys, or trenches.