L Left to right: A tsunami begins with an undersea earthquake. The force jolts the ocean floor, displaces water, and produces waves. Slowing as they enter shallow water, the waves begin to pile up. (This is similar to a rug being rumpled against a wall.) The waves can rise 100 feet (30 1/2 meters) or higher. Sweeping onshore, the fearsome walls of water wreck property and take lives. Warning centers around the Pacific Ocean detect approaching waves and alert people so they can head for high ground.