LONDON — For three decades it was a mystery that seemed to defy belief.

Bright orange plastic novelty phones shaped like the grumpy cartoon cat Garfield kept washing up on the rocky Atlantic shoreline of Brittany, in western France.

Over the years, locals have picked up hundreds of pieces of the phones, including paws, headset cables and even Garfield heads, forever fixed in his familiar smirk. But nobody knew exactly where they came from.

Last week, volunteers cleaning the beaches solved the puzzle: The source of the Garfield phones was a long-lost shipping container, nestled in a rocky sea cave.

“This waste is over 30 years old, and we are still finding bits,” Fabien Boileau, the director of the Iroise Marine Natural Park, told the news site FranceInfo, citing it as an example of plastic debris that never fully breaks down and contributes to ocean pollution.