So many died they were lined up in rows along the riverside.
Rescue teams described a "scene of carnage" as they battled to save scores of common dolphins which had become trapped in a Cornish river creek off the Fal Estuary, near Portscatho.
Fifteen kilometres (9.3m) away in the Helford River, near Gillan, another group had also become stranded. In total 26 animals died.
And in Falmouth Harbour on Monday evening a school of up to 10 young dolphins was spotted unusually close to the shore.
But the reason behind their strange behaviour remains a mystery.
Initial tests showed the dead animals appeared to have been well-fed and there were no obvious signs of disease or poisoning.
Questions are now being asked over what led to the fatal strandings, which experts believe were the worst ever seen in the UK.
There are on average 45 whale and dolphin strandings every year in the UK.
But it rarely happens en-masse.
Sarah Dolman, science officer for the Whale and Dolphin Conservation Society, said most of the stricken dolphins were juveniles, less than five years old.
They were common dolphins, found in large schools and native to UK waters.
She said the stranding of so many in two different rivers was unprecedented.
"The animals that were post-mortemed were all healthy. Over the next few months there will be all sorts of tests," she said.
"Noise is certainly one of the possibilities. Around the world naval sonar has in other cases caused animals to strand."
Ms Dolman said it was possible a predator such as a Killer Whale could have frightened the dolphins.
But she said this was an unlikely cause, as the dolphins were beached over such a wide coastal stretch.
The Royal Navy originally said none of its ships had been in the Falmouth area over the weekend but later admitted it had been carrying out a survey in the area at the time of the strandings.
But Trevor Weeks, from the British Divers Marine Life Rescue, said many disturbances could have unsettled the animals.
"Any sort of underwater shockwave or noise could cause dolphins to strand, it doesn't necessarily have to be a navy thing. Strandings have always occurred.
"There is factual data out there that naval sonar can cause strandings."
Liz Sandeman, director of operations at Marine Connection, said although sonar had been in existence for decades, evidence had only emerged in the last 10 years that it can cause dolphins and whales to strand.
Mr Weeks said some of the dolphins which were found out of the water had been saved by being put on to pontoon systems and towed slowly out to sea.
Others still in the shallow rivers were gently coaxed back into deeper waters.
But many were already dead when rescuers arrived, he said.
Sue White, also from the British Divers Marine Life Rescue, said: "They were scattered around a large area...something must have frightened them."
Another theory is that a few dolphins may have got into trouble while feeding and their distress calls lured others into the inlets.
"We have been told there was large algae blooms which the fish come and feed on and then the dolphins come in for a feeding frenzy," Ms White said.
Ms White said the recovery operation had been a tough experience for the rescue teams.
"We get the odd dolphin or whale stranded down there, but nothing on this scale.
"When you see the picture of the dolphins lying there dead, and there's no reason for it, it's very upsetting."