How Dangerous Are Tsunamis? Activity Source: Adapted with permission by Geological Society of America. Imagine playing beside the ocean, when suddenly, the water drops. Where the water used to be, there are wriggling fish and ribbons of seaweed. What do you do? You could be seeing the first sign of a tsunami ─ a long wave formed in the ocean when the sea floor moves suddenly. Most tsunamis happen because of large earthquakes on the ocean floor. [Read More]