Sea turtles have puzzled the scientists since…ever. It is especially the case of the green sea turtles (Chelonia mydas), gentle offshore sea grass eaters, which got their name from their greenish meat and fat, once used for soup.

Thousands of sea-turtle hatchlings head towards the sea and after that…total mystery. You see them at a size of 2-inch (5 cm) in length and after five years they reappear closer to the shore, out of nowhere about the size of a dinner plate; they live up to 80 years and reach almost 395 kg (871 pounds) in weight with a shell which is over 1 m (3 ft) long. But individuals of an intermediate size have, to their shame, never been handled even by turtle specialists.

A new study has solved the long-standing mystery coming with clear proofs that the juvenile green sea turtles hide out in the open ocean, feasting on jellyfish and other sea animals (like most adult sea turtles do). The peaceful vegetarians are in fact carnivorous as juveniles.

"This has been a really intriguing and embarrassing problem for sea-turtle biologists, because so many green-turtle hatchlings enter the ocean, and we haven't known where they go," said co-researcher Karen Bjorndal, a zoologist and director of the University of Florida's Archie Carr Center for Sea Turtle Research.

"Literally, when green turtles run off their nesting beach and into the ocean as little hatchlings, they disappear. And nobody sees them again [for years]," Bjorndal told LiveScience.

The research team collected shell samples from 44 green sea turtles at a location near Great Inagua Island in the Bahamas and made an analysis of heavy and light stable isotopes of carbon and nitrogen from both the oldest (earliest-grown) and newest portions of the shells.

The isotopes record an animal's food (carnivore or herbivore) and the exact place in the ocean that provided the creature with food. This is how it clearly appeared that the green sea turtles spent their "missing" years in the deep ocean, feeding as carnivores, before approaching the shallow waters to feast on sea grass.

The discovery is huge for the conservation of the green turtles, as "you can't protect a species if you don't know where it is." said Bjorndal.