The European eel is a surprisingly enigmatic creature. Its breeding behaviour in particular has confounded people for over 2,000 years. To some, its proliferation seemed inexplicable. The great Greek thinker Aristotle, for example, believed that eels spontaneously emerged from mud. A few hundred years later, Pliny the Elder had his own imaginative ideas about eel reproduction – he thought they bred by rubbing their bellies against rocks.