Which sea has no coastline at all?

The Sargasso Sea, in the middle of the North Atlantic, is the only sea on Earth defined by currents rather than land. Four of them circle it - the Gulf Stream to the west and three others closing the loop - and they hold a slowly rotating oval of unusually clear, warm, still, deep blue water roughly the size of the continental United States.

It is named for the sargassum weed that floats in mats across it, a seaweed that lives its whole life adrift and never attaches to anything. Columbus recorded it in 1492 and his crews took the weed as a sign of nearby land, which it was not. A whole ecosystem lives in those mats, including species found nowhere else, and it is where European and American eels come to breed - which is the other reason it turns up in this library.

The stillness that concentrates the weed also concentrates everything else. It sits at the heart of the North Atlantic gyre, and it is one of the ocean's great accumulations of floating plastic - the same physics that gathered the sargassum for the eels gathers everything we have dropped in the Atlantic.