Eels, creatures that once existed alongside dinosaurs, are still one of the most mysterious creatures known to mankind.

Snake-like and nocturnal, ancient Egyptians believed that eels were born by the sun’s rays hitting the Nile. Meanwhile, Aristotle concluded that they leak out of mud and rainwater, Brooke Jarvis reported for The New Yorker.

What do we know about eels and their lifecycles?

Adults eels travel distances anywhere between 3,000 to 6,000 miles to a spawning area, “where they grow for decades — reaching weights of more than 20 kilograms — only to return to the sea, where they spawn, die and sink into the abyss,” wrote Jarvis.

Fishermen in Maine have made selling eels a multimillion dollar business. In Japan, unagi, the freshwater eel, is an everyday staple. Per PBS, these sea creatures can be found all around the world, in both freshwater and saltwater bodies.

“But today, risk of over-fishing and the presence of dams and other obstacles that prevent eels from reaching their oceanic spawning grounds pose new threats,” PBS stated.

The international Union for Conservation of Nature has classified eels as “critically endangered.”

What is the mystery behind eels?

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With that said, these animals are still considered mysterious. Now, scientists at least know where they originate from.

“They’ve managed to follow them back to approximately where they’re spawning,” said Sarah Coney, an aquatic biologist with the Catskill Regional Invasive Species Partnership, as per Sierra Club. “But nobody has ever seen it.”

They’re an important food source globally, yet scientists can't figure out how to breed them. According to Nature, they “spawn in remote and nutrient-poor places.”

The report further adds that the economic and cultural importance behind the eel as well as its unique life cycle is a great argument in support of preserving the species.

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