It started in the 1980s: bright orange fragments of Garfield novelty phones were spotted washing up onto a stretch of coastline in Brittany, France. For the past 30 or so years, locals have been finding coiled phone cords, receivers and feline heads strewn across the shore. Beach cleaners would pick them up. Inevitably, the cat phones, with their mocking smirks, came back.

The source of the pollution was a disconcerting mystery. Claire Simonin-Le Meur, president of the environmental group Ar Viltansoù tells Meagan Flynn of the Washington Post that activists worried the phones were drifting up from a lost shipping container at the bottom of the ocean, where the plastic Garfield merchandise could be contaminating the marine ecosystem. After years of speculation, the mystery of the wayward Garfield phones was recently solved. But the concerns around their environmental impact remain.

The breakthrough in the case came when the publication FranceInfo reported on the Garfields as part of a campaign called Pollution Alert. This caught the attention of a local farmer named René Morvan, who met Simonin-Le Meur on the beach and told her he knew where to find the cartoon cats. According to Flynn, he said that in the mid-1980s, he’d observed the orange phones dotting the beach after a storm. He and his brother decided to investigate, combing the rocky area in the hopes of discovering the source of the strange deposits. Eventually, Morvan said, they found it: a metal shipping container, stuffed with Garfield phones, tucked deep in a sea cave.

For most of the year, the cave is rendered inaccessible by the tide, reports Palko Karasz of the New York Times. But last month, volunteers were able to make it inside. As they climbed up to the entrance of the cave, they stumbled upon bits of Garfield scattered across the rocks. They hoped that once they entered the cave, they would find the shipping container still brimming with phones—potential debris they could prevent from entering the ocean environment. “But that was unfortunately not the case,” Simonin-Le Meur tells Flynn. “What we found was the remainder of the shipping container. And it was empty.”

In France, the Garfield phones are a symbol of a much broader issue of plastic pollution; it has been estimated that between 4.8 and 12.7 million tons of plastic enter the ocean each year. Particularly disconcerting is the fact that the phone parts seem to be largely intact 30 years after they first started to appear, demonstrating how persistent plastic pollution can be in the environment. Marine animals eat plastic debris and become entangled in it. And even when plastic objects degrade, they don’t fully disappear; instead, they break down into tiny “microplastics” that get consumed by marine life and make their way up the food chain to humans, the health risks of which are still unclear.

So while environmentalists have resolved the mystery of the Garfield phones, they are not feeling celebratory. “[T]he bulk of the phones are already gone, the sea has done its job for 3o years,” Simonin-Le Meur tells Le Monde, per Times’ Ashley Hoffman. “We arrive after the battle.”