ROME — 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 rough seas were making it difficult to understand even how many whales were still alive, with other experts telling Italian media that only one was breathing.

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.

Speaking to Italy’s Sky TV, he said the incident may have been caused by noise from military exercises or surveys for underwater mineral deposits that can confuse whales and interfere with their communication.

“When a sperm whale washes up on the beach it’s usually already dead,” he said. “When an entire pod ends up on the shore it’s difficult to think that the cause is natural, otherwise it would happen more often.”

Scientists had taken tissue samples from the whales to try to determine the cause of the beaching while authorities were working on the difficult job of burying the huge carcasses.