tl:dw: We knew that's where baby eels came from but we didn't know how the adults got there or what the larvae looked like. Baby eel larvae was misidentified as another species and adult eel can take up to 18 months traveling at the bottom of the ocean to get there, during which time they grow their gonads which was another mystery.
When it’s time to mate, eels are very determined to make it to their breeding site at the Sargasso Sea. The Sargasso Sea, a two-million-square-mile span of ocean, is the site in which all freshwater eels mate
It’s way the hell down there in the article, though. Apparently they travel to freshwater as larva.