Italy ? A pod of sperm whales beached on Italy’s southern coast and at least five died in what experts said Saturday was a rare and puzzling mass beaching for such a large species.

Nine whales measuring up to 40 feet in length were stranded Thursday on a beach in Puglia, the heel of boot-shaped Italy.

Only two managed to swim back to deeper waters and at least five were dead by Saturday, said Nicola Zizzo, one of the veterinarians caring for the animals. He said officials were considering euthanizing the last two whales still trapped in high waves just off the beach.

The sperm whale is the largest of all toothed whales and is considered a vulnerable species.

While similar mass beachings are more common in the oceans, they are extremely rare in the Mediterranean, occurring once every 150-200 years, said Giuseppe Notarbartolo di Sciara, a marine biologist and head of a conservation group.