Echosounders measure the depth of water by timing how long it takes sound to travel from a pinger on the ship down to the seafloor and back. Modern echosounders create a fan of sound beams reaching out on both sides of the ship to give a wide band of measurements. By “mowing the lawn” the ship can produce a complete map from several passes. The maps show volcanoes, geological faults, sediment-filled valleys and long, thin ridges built from small volcanic cones. They are crucial in working out the evolution of the ocean floor.