The eel is a long, thin bony fish of the order Anguilliformes. Because fishermen never caught anything they recognized as young eels, the life cycle of the eel was a mystery for a very long period of scientific history. Although more than 6500 publications mention eels, much of their life history remains an enigma.

Source : Wikipedia

The main spawning-grounds for eels worldwide have now been identifed - for example, for both the European eel and the American eel it's the Sargasso Sea, and for the Japanese eel it's an area near the Mariana islands.

But spawning has never been observed, and the fertilisation methods are currently unknown.

In addition, the means by which eels accurately navigate to and from the spawning grounds - sometimes up to 6,000km - are also unknown.