Earlier this month, the California coast was hit by a Kelvin wave, a deep ocean wave that can raise sea levels, but to most people, it wasn’t immediately visible. Named after Lord Kelvin, the mathematician who discovered them in the late 19th century, these waves don’t roll and crash along the shore like a tsunami or hurricane. They are slow-moving, mostly flat, waves of warm water that can stretch thousands of miles across the Pacific Ocean.

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