Nobody has ever watched a European eel being born. Aristotle decided they came out of the mud. Freud, as a young researcher, cut open hundreds of eels looking for testes and never found any. In the early 1900s a Danish biologist, Johannes Schmidt, went at it from the other end: he spent years trawling the Atlantic for eel larvae and noticed that the further west and south he caught them, the smaller they were, until the smallest of all turned up in the Sargasso Sea. That is still most of the evidence. No one has found an eel egg there or seen a pair spawning; eels have since been tracked heading that way, and the rest is inferred. It is how you find where a rumour started. You can’t hear the first telling, so you follow it back to the place where it is smallest.