In the middle of the Atlantic Ocean, there’s an enormous patch of seaweed that’s perplexed sailors for centuries: the Sargasso Sea. This strange place is where American and European eels go to breed. Once born, the little eels — called elvers — have to venture toward land.

American eels live out their lives — which can be more than a decade — just off the eastern seaboard. Their cousins across the pond live everywhere from Scandinavia to North Africa. Then, at the ends of their lives, bot