The mystery of how eels migrate from European rivers to their spawning grounds in the Sargasso Sea is a century-old puzzle. Now, data from tagged eels suggest the critically endangered animals use a variety of routes on their final, 4800-kilometer journey, which sometimes takes as long as a year, BBC reports. Each autumn, adults swim from their freshwater enclaves to a swath of the Atlantic Ocean, where they mate and die, but not before laying millions of eggs each that hatch into young eels who then make the return journey to Europe. The population of European eels has plummeted in recent decades, but efforts to help boost their numbers have been stymied by their complex and mysterious life cycle. Researchers hope the new data, published today in Science Advances, will help them find ways to help more eels survive—and reproduce.