The ocean’s waves range from tiny, fleeting ripples to tremendous, booming curls. When we observe an ocean wave, what we see is a disturbance in the water caused by the transfer of energy. Typically, this energy comes from the wind and is transferred to the water. Waves are also caused by events such as undersea earthquakes, calving glaciers, and landslides.

A drawing of a typical wave features a top and a bottom. The highest part of a wave is called its crest; the lowest part is a trough. Wavelength is measured from crest to crest or trough to trough. Wave height is measured from crest to trough.

Waves form when wind energy in the atmosphere is transferred to the sea surface. As the wind continues to blow, the sea surface becomes rougher. Waves become more organized and begin traveling in one direction. Stronger winds, such as those that blow during storms, tend to produce waves with longer wavelengths, which move faster than waves with shorter wavelengths. Waves organize themselves by wavelength, forming a series of waves that pass in a regular pattern, known as a wave train.

But sometimes giant waves seem to appear out of nowhere. Sailors throughout history have described these sudden “walls of water” out at sea. These rogue waves can be incredibly huge and extremely dangerous. Rogue waves are often steep with very deep troughs. Scientists do not completely understand how rogue waves form. One explanation is that wave trains travel thousands of miles across ocean basins, encountering other wave trains as they move. When the crest and trough of two different waves meet head-on, they can cancel each other out, resulting in a flat sea. But when two crests run into each other, they form a huge wave that is the sum of both wave heights—a rogue wave that towers above other nearby waves.

Scientists and sailors have noted that rogue waves are more likely to occur when wave trains run into fast ocean currents. For example, off the coast of South Africa, wave trains frequently encounter the strong Agulhas current, causing the waves to become even steeper. Scientists continue to study rogue waves, which remain very difficult to predict.

One concern is that rogue waves may become more common as Earth’s climate changes. Our planet is in a warming period, which means there is more energy in both the atmosphere and ocean. Scientists predict that over the coming decades, there will be more frequent storms with higher wind speeds. More energy may result in larger waves and therefore even larger and more frequent rogue waves, which can lead to more destruction and lost ships at sea.