Eels were once a common sight in European streams and rivers. But their breeding habits were a mystery, for the eggs and baby eels were never seen. Then in the 1920s, experts at last discovered that the eels swim downriver, and make their way across the Atlantic to breed. They spawn in the warm waters of the Sargasso Sea off the coast of Florida. When the eggs hatch out, the young eels drift slowly back to Europe, and swim upriver.