Aristotle examined eels, found neither eggs nor milt in a single one, and concluded that they are born from mud. The guess held for two thousand years, because nobody could offer better: eels appeared in rivers, grew, and left again, and no fisherman in Europe had ever seen a young one.
The answer came from a Dane. Between 1904 and 1922 Johannes Schmidt crossed the Atlantic again and again, netting eel larvae and noting where they were smallest. The trail narrowed with every voyage and ended in the Sargasso Sea — a windless patch of ocean west of Bermuda with no shore of any kind. That is where they spawn. Every eel in the Loire, the Volga and the Nile began there.
The journey back
The larvae are transparent and shaped like willow leaves. They drift on the Gulf Stream for one to three years, reach the European shelf, become glass eels, push up rivers — sometimes climbing wet rock to do it — and spend up to twenty years growing in fresh water. Then they turn silver, their eyes enlarge for deep water, their guts shut down, and they swim six thousand kilometres back without eating.
The spawning itself has still never been observed. Not once, by anyone, in the wild. We know where it happens only because of where the smallest larvae are found.