The longest ocean waves are known as "tsunamis." Tsunamis are a type of seismic sea wave caused by the displacement of a large volume of water, usually resulting from undersea earthquakes, volcanic eruptions, or landslides. They can also be generated by meteorite impacts in the ocean.
Tsunamis can travel across entire ocean basins, covering vast distances. While most ocean waves are characterized by relatively short wavelengths, tsunamis have extremely long wavelengths that can span hundreds of kilometers (or miles). However, their amplitudes (heights) are generally low in deep ocean waters, making them difficult to detect.
When tsunamis approach shallow coastal areas, their long wavelengths cause the sea level to rise dramatically, resulting in devastating and destructive effects. As the leading edge of the tsunami wave reaches shallower water, it slows down, causing the trailing portion to catch up and leading to a significant increase in wave height. This process is known as "tsunami amplification."
It's important to note that while tsunamis have long wavelengths, they are distinct from typical ocean waves that we encounter near shorelines, which are primarily influenced by wind and have much shorter wavelengths.