This sea monster is a tsunami (tsoo-NAH-mee). That’s Japanese for “great harbor wave.” Though sometimes called “tidal waves,” tsunamis have nothing to do with tides. Usually an undersea earthquake starts a tsunami’s waves rolling across the ocean. If you’ve ever tossed a pebble into a pond, then watched ripples spread out over the surface, you’ve seen this principle at work.

About four out of five tsunamis happen within the “Ring of Fire,” a zone of frequent earthquakes and volcanic eruptions roughly matching the borders of the Pacific Ocean. Along the ring’s edges, giant slabs of the earth’s crust, called tectonic plates, grind together. Sometimes the plates get stuck, and pressure builds. Then, the plates can suddenly come apart and slam into a new position. The jolt causes an earthquake. If an earthquake lifts or drops part of the ocean floor, the water above it starts moving too. This triggers a tsunami.

A tsunami can race across the ocean at 500 miles (805 kilometers) an hour. Oddly, in deep water its waves are only a few feet high. But when the waves approach shore, they increase in energy and height. Often before a tsunami hits, there is a giant vacuum effect, and water is sucked from harbors and beaches. People see the bare sea bottom littered with flopping fish and stranded boats. That is because waves are made up of crests, or high points, and troughs, or dips between crests. When a trough hits land first, the water level drops drastically. Usually another wave blasts ashore about 15 minutes later, then another and another—for two hours or more.

Tsunamis have killed more than 50,000 people in the past century. To save lives, scientists established the Pacific Tsunami Warning System, based in Hawaii, in the U.S.A. Its network of earthquake detectors and tide gauges detects quakes that may cause a tsunami.

We can’t tame the tsunami. But we can learn when it’s coming and escape the sea monster’s fury.

Text by Jerry Dunn