If the ocean waves begin to recede unexpectedly, says the folklore, head for high ground. That disappearing water can mean that a tsunami is on its way. Tsunamis are waves that can be caused by landslides and volcanic eruptions, but they're most famously spawned by underwater earthquakes. The shaking pushes the water to and fro, and the ripple effect causes waves that may travel hundreds or even thousands of miles.
Tsunamis can be dozens of feet tall, but in deep water, the waves may not be apparent on the surface. When these waves approach land, the upper portion of the waves is faster on the move than the lower portion. Then massive water walls crash into (and over) seawalls and entire cities with the violence of a flash flood combined with the power of several raging rivers. Concrete buildings may survive ... or they may crumble beneath the water's force. Your only real hope of survival is high ground.
In 2004, an earthquake of around magnitude 9.1 occurred in the Indian Ocean near Sumatra. The resulting tsunami affected 14 countries and blasted some with waves nearly 100 feet (30 meters) tall. Unrelenting water scoured entire towns, pushing buildings as though they were toys and drowning tens of thousands of people. When the waters finally dried, more than 230,000 people were dead, making this one of the worst natural disasters in human history.