Tsunamis are large waves of water usually triggered by an earthquake, explosion or other event displacing a large amount of water. Out in the open ocean, tsunamis are typically harmless and unnoticeable to the naked eye. When they start, tsunami waves are small and wide the height of the waves can be as small as a foot, and they can be hundreds of miles long and move very quickly, so they can pass practically unnoticed until the get to shallow water closer to land.