Johannes Schmidt spent decades in the early twentieth century towing nets across the Atlantic and mapping where the smallest eel larvae were found. The smallest turned up in the Sargasso Sea, so that region was concluded to be the spawning ground. No adult eel had been observed there, and no eggs were collected.
The inference held for a century as an unusually well-supported gap. Then in 2022 a study reported satellite tags on eels released from the Azores that continued into the Sargasso Sea, providing the first direct tracking of adult European eels reaching the presumed spawning area.
The species is in serious trouble regardless. Recruitment of glass eels to European coasts fell dramatically from the 1980s, and the IUCN lists Anguilla anguilla as Critically Endangered. Barriers to migration, habitat loss, parasites, fishing and changes in ocean currents have all been proposed, and no single cause accounts for the decline.