Eels are guided by Earth’s magnetic field

FROM ARISTOTLE to Sigmund Freud, eels’ reproductive habits have puzzled observers of the natural world. In a life-cycle the opposite of a salmon’s, they grow from youth to maturity in rivers and ponds and then go to sea to spawn. Exactly where they do this spawning, though, was a mystery—until, a century ago, a Danish marine biologist called Johannes Schmidt painstakingly trawled eel larvae from the depths of the Atlantic Ocean and found that they got smaller and smaller until he arrived at the Sargasso Sea, which thus seemed to be European eels’ fons et origo.