Many people in Japan eat eels. But for a very long time, nobody knew where eels were born. Aristotle thought eels came from mud. In 2009, Japanese scientists finally found Japanese eel eggs in the ocean.

The place was west of the Mariana Islands, more than 2,000 kilometers south of Japan. Adult eels swim there from Japanese rivers to lay their eggs. Their babies come back to Asia on ocean currents.

Although eels have been eaten for centuries, where they are born remained a mystery for more than 2,000 years. Aristotle could not find eggs inside eels, so he believed they came from mud. The reason for the mystery is that eels do not become mature in rivers; they become adults only after traveling far out to sea.

In the early 20th century, a Danish scientist collected baby eels in the Atlantic and traced European eels to the Sargasso Sea. For Japanese eels, researchers at the University of Tokyo followed tiny larvae for many years and finally collected eggs west of the Mariana Islands in 2009. Today, however, the Japanese eel is listed as endangered.

Freshwater eels have puzzled scientists for more than two thousand years. Aristotle, finding no eggs inside them, proposed that eels arose from mud, and the young Sigmund Freud reportedly dissected hundreds of eels in Trieste without clearly locating male reproductive organs. The difficulty arises because eels mature only after leaving rivers on a long oceanic migration. In the early 1900s, Johannes Schmidt traced ever-smaller larvae westward across the Atlantic to the Sargasso Sea, yet direct evidence of adults arriving there came only in 2022, when at least one of 26 satellite-tagged European eels was recorded reaching the spawning area.

For the Japanese eel, Katsumi Tsukamoto's team at the University of Tokyo narrowed down the spawning site step by step and, in 2009, collected wild eggs west of the Mariana Islands, more than 2,000 kilometers south of Japan. Spawning appears to occur around the new moon near a chain of seamounts. Even so, spawning itself has never been directly observed, and why eels travel so far to breed is still a matter of hypotheses. Meanwhile, the species is endangered.