Experts are trying to figure out what caused loads of dolphins to swim up a river in Cornwall where they became stranded and died.
Rescuers helped seven animals back out to sea but 26 died with two so poorly they had to be put to sleep.
The animals appeared to have been well-fed and there were no signs they had been poisoned or had a disease.
Marine experts are continuing to search the area near the Percuil River in case there are more dolphins in trouble.
Dolphin deaths possibilities
They were chasing fish and got caught out by low tide OR
They were scared by something so swam up the river OR
They were confused by ships' navigation sound
The problem was made worse becasue other dolphins were attracted to the river by the distress calls of the stranded animals.
British Divers' Marine Life Rescue charity spokesman Tony Woodley said this was the biggest mass stranding of marine life in the UK for many years.
"We haven't seen a stranding anywhere near this scale since 1981 when pilot whales were beached on the east coast.
"This is extremely rare."