The Sargasso Sea has no coastline. It sits inside the North Atlantic gyre, bounded by the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south, a clockwise vortex driven by wind and the Coriolis effect. Roughly 1,100 by 3,200 kilometers, between 20 and 35 degrees north with Bermuda near its western edge, it gathers floating mats of sargassum, a brown macroalga that lives and reproduces at the surface. Its water is clear to about 60 meters because sinking surface water keeps nutrients from welling up, making it a marine desert with golden oases. Portuguese navigators named it, and Christopher Columbus’s crew neared mutiny in 1492 as he feared the weed would foul his rudders or hide reefs, echoing the poet Avienus’s account of the Carthaginian explorer Himilco.

The real trap was wind, not weed: the sea overlaps the horse latitudes, where sinking air from Hadley cells creates high pressure and dead calms. That stillness makes it a nursery. Johannes Schmidt’s Dana expeditions in the 1920s traced European eels, American eels, and American conger eels back to it, and European eel larvae drift about three years to reach rivers, living there over a decade before returning to spawn and die. Young loggerhead turtles shelter in the mats, and the sargassum fish, Histrio histrio, climbs through the weed on modified fins. The Global Ocean Sampling Expedition’s metagenomics found millions of new genes, and 1975 estimates put the sargassum’s oxygen output at about 2.2 billion liters per hour.

• Jules Verne and Ezra Pound’s poem Portrait d’une Femme used the sea as an image of entrapment and stagnation.
• The same currents that gather the weed feed the North Atlantic garbage patch.
• Bacteria that eat plastic may carry absorbed pollutants up the food web through biomagnification.
• Overfishing, shipping lanes, possible oil spills, and deep sea mining add pressure.
• The Azores, Bermuda, Monaco, the UK, and the US formed the Sargasso Sea Commission in 2014, but the high seas location limits enforcement.