For more than 30 years, bright orange "Garfield" phones have been washing up on the French coast to the bemusement of local beach cleaners, who have finally cracked the mystery behind them.

Locals had long suspected a lost shipping container was to blame for the novelty landline phones, modelled on the prickly feline cartoon character, that have plagued the northern Finistere beaches for decades.

"Our association has existed for 18 years and in that time, we have found pieces of Garfield telephones almost each time we clean," said Claire Simonin, the head of local beach cleaning group Ar Viltansou in Brittany.

But it wasn't until a local resident revealed that he had discovered the container after a storm in the 1980s that they were finally able to locate it — wedged in a partially submerged cave only accessible at low tide.

"He told us where it was ... it was very, very dangerous," Simonin told AFP after an expedition to track it down. "We found this incredible fissure that is 30 metres deep and at the very bottom, there were the remains of a container."

"Under the boulders in front of the entrance, we found 23 complete handsets with electronics and wires. They were everywhere," she added.

But the mystery is not fully solved.

"We have no idea what happened at the time: we do not know where it came from, what boat," said Fabien Boileau, director of the Iroise Marine Nature Park in Finistere.

"And we don't know if several containers fell into the water, or only one."

The dry-witted Garfield, first dreamed up by illustrator Jim Davis in the late 1970s, has since spawned a television show, a film series starring Bill Murray as the voice of the titular cat, and a merchandising empire worth hundreds of millions of dollars annually.