Back in 1997, John Lambert, a resident of Suffolk, England, was installing a new garden fence when he struck a hard object. He dug the rock-like structure up, and it appeared to be some type of bone. Rather than call an expert or turn it in, Lambert stuck the 15-pound, 16-inch long mystery object in his shed, the Daily Mails reports. There it sat for 14 years.

Recently, the now-retired Lambert finally remembered the bone and decided it was time to act on it. He dug through the shed, found the bone and called the nearby Ipswich Museum. The specialists asked him to bring the bone in, and were shocked to find that it belongs to a 250 million year-old pliosaur, a sea-faring predatory reptile that could grow up to 65 feet long.

“‘Life get’s on top of you doesn’t it,” Lambert said, explaining his long delay on turning in the fossil.

The mystery, however, deepens. As far as experts know, pliosaurs only lived in waters around Africa, Australia and China—not Great Britain. How did the fossil come to be buried in Lambert’s garden? Perhaps it came down from northern seas with glacial clays, one curator suggested to the BBC. Or someone else could have brought it to Lambert’s property years before, and lost it, or buried it, until Lambert had the luck to dig it back up.

More from Smithsonian.com:

Death of a Sea Monster

Our Nemesis’ Front Yard