What is unusual about the seaweed in the Sargasso Sea?

It never attaches to anything. Sargassum in the open Atlantic is holopelagic - it lives, grows and reproduces entirely adrift, and has done for so long that the two species involved appear to have no attached form at all. Almost every other seaweed spends its life anchored to rock.

An entire ecosystem lives inside the mats. Sargassum fish, crabs and shrimp are camouflaged to match the weed to an almost absurd degree, and sea turtle hatchlings spend their missing early years - the lost years, unobserved for decades - sheltering in it. Columbus recorded the weed in 1492 and his crews took it as a sign of nearby land, which it was not.

There is now far too much of it. Since 2011 an enormous belt of sargassum has formed across the tropical Atlantic, thousands of kilometres long and visible from orbit, and it washes ashore in the Caribbean and West Africa in quantities that smother beaches, rot, release hydrogen sulphide and wreck tourism and fisheries. Nutrient runoff from the Amazon and the Congo and changes in ocean circulation are the leading explanations, and it appears to be a new normal rather than an anomaly.