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Scientists are trying to find out why 33 whales got stranded and died off the coast of County Donegal in Ireland.
It's thought some of the pilot whales had been ill for a while before being washed up on Rutland Island.
The Irish Whale and Dolphin Group said it's one of the biggest mass deaths of whales in Irish history.
The group's worried the whales got lost after being confused by military sonar equipment, but the Royal Navy says its closest ship was 50 miles away.
A Royal Navy spokeswoman said there was no way the sonar equipment could have affected the whales over such a long distance.
It's thought the group of whales may be the same one that was spotted in the Outer Hebrides at the end of October.
Dr Simon Berrow from the Irish Whale and Dolphin Group, said: "Thirty or 40 pilot whales were spotted off the Outer Hebrides at South Uist last week.
"It looked like they were going to strand. It was bad weather. They were not seen again."