Rows of tall buildings channel the breeze, turning streets into wind tunnels and creating whirlwinds. A similar effect underwater may be deadly.

Tidal currents can produce giant whirlpools. Some, like the famous Maelstrom off the Norwegian coast, have been known as shipping hazards for centuries. Their destructive power feeds mythology; Maelstrom is the home of the mythical Kraken, which drags ships down, while regular whirlpools in the straits of Messina are blamed on the fearsome Charybdis.

Underwater whirlpools pose an additional danger because they cannot be seen. Known as deep eddies, they are produced by ocean currents flowing around terrain features like seamounts (submerged mountains). They do not affect surface shipping, but are a hazard to underwater navigation, as shown in 2014 when Chinese Submarine 372 started plunging unexpectedly into an ocean trench.

“It was like a fast car suddenly running over a cliff,” submarine commander Captain Wang said later.

As the submarine descended, a pressure pipe burst and water poured in. Only prompt action in closing valves and releasing gas to increase buoyancy prevented Submarine 372 from dropping below its operating depth and being crushed by the pressure.

Since the accident, researchers from the Guangzhou Institute of Oceanology have mapped several deep eddies in the Xisha Trough, an area of abyssal ocean off Hainan. Further surveys will reveal all the lairs of these monsters which menace unwary submariners, as well as improving our understanding of how water and sediment mix in the deep oceans.