When you watch waves crash on the beach, you’re seeing the visible end result of many invisible forces.

Waves are formed near or very far from shore, usually by wind—the first invisible force.

As wind blows over the sea, surface friction transfers its energy to the water. How fast, how far, and for how long the wind blows determines how much waves grow.

When the wind stops blowing, the waves stop growing—but they don’t stop traveling.

A wave is not actually water traveling but instead another invisible force: energy traveling through the water. When a wave passes, water molecules lurch forward then back, coming to rest in nearly the same place. But that movement transfers energy to the next water molecule...and so on.

Uninterrupted, this wave energy can travel thousands of miles through open ocean, sometimes for more than a week, before finally reaching a shoreline.

As the water grows shallower, another invisible force comes into play: friction against the ocean bottom, which slows the base of the wave.

But the top of the wave keeps moving at its original speed, eventually crashing forward over the wave base. The shape of that wave break is determined by the unseen topography below it.

In some places, deep undersea canyons channel and concentrate wave energy as it reaches shore, to form some of the largest waves in the world.

Spectacular sights, formed out of sight.