The World’s Largest Waterfall is Under the Sea, and a Barometer for the Planet's Health

The largest waterfall in the world, a cascade of cold, dense water, lies deep under the surface of the ocean. The water doesn't fall over an escarpment or cliff, but it's a waterfall just the same: As cold water hits warm water, it plunges in a cataclysm deep into the abyss. Instead of falling over rock, the water is falling over other water.

This underwater downward rush of water is the Denmark Cascade. Cold water coming south from Greenland hits warmer water coming north from Iceland. Where the two water masses meet, the cold mass falls 11,500 feet straight down in a massive downwelling that dwarfs any waterfall on land.

The cascade flows along the length of the boundary at an astonishing rate of 175 million cubic feet a second, almost 200 times greater than Inga Falls on the Congo River. It is one of the largest and most powerful events on Earth, and it happens continuously. It's happening as you read these words.

Its importance goes beyond its awesome scale. The downwelling of cold water and the upwelling of warm is the basis for deep sea current, which has gone on for billions of years. The process is biologically important as nutrients are exchanged between the ocean's surface and its depths. Billions of marine organisms live out their life cycles in rhythm with the vertical currents like the Denmark Cascade.

The convection cells in deep ocean waters are driven by differences in density and salinity. Cold, salty water is denser than warmer water with less salt. Downwelling and upwelling constantly circulate ocean water in immense circular flows

The deep ocean current, its eddies and cataracts, also are the basis for healthy populations of fish and with them the very life of the commercial fishing industry in polar regions.

These cascades can also occur in warmer waters. A team from Barcelona studied one in the Mediterranean in 2008. They took data from the Gulf of Leon off the southern coast of France.

What they found was worrisome. They monitored a cataract, and it seems to be slowing down over time. Is it possible a changing climate is affecting the very deep sea current that supports life throughout the world?