More than a dozen dolphins died over the weekend after 125 of the mammals became trapped in shallow waters near Cape Cod.

Animal rescuers managed to save about 100 of the white-sided dolphins from the shallow waters off the coast of Massachusetts, but an estimated 13 ultimately died. One of the animals had to be euthanized, according to the International Fund for Animal Welfare.

The Fund, a non-profit organization dedicated to animal welfare, led the rescue effort and said the mass stranding was the largest it had seen in its 26 years of working in the Cape region.

Rescuers said there was no single reason why the dolphins were stranded other than location. Cape Cod has been identified as a global stranding hotspot due to the shape of its shores and tidal movements. WSFA12 reports.

On Saturday, rescue workers in Massachusetts found a group of ten stranded dolphins swimming in extremely shallow water. They managed to guide the dolphins back to deep water.

Afterward, a second group of 25 dolphins was spotted swimming dangerously close to shore in Eastham, according to the fund, and rescuers again attempted to herd them back to safer waters.

Most of the dolphin deaths occurred Friday at a place called “The Gut,” near the Herring River in Wellfleet. Marine experts say “The Gut” is notorious for strandings.

A resident near The Gut discovered a dying dolphin lying on its side and described her anguish at its suffering to USA TODAY.

“I jumped in the water crying because I could hear it suffering. It was making little noises and I wasn’t expecting to hear that and that just tore me apart,” Marina Costello, 54, told the outlet. “After it died, I was just heartbroken to see such a magnificent, majestic animal die, it was a tragedy.”

She tried to save the dolphin herself, but the animal weighed 136 kilograms and was far too big for her to move alone.

Rescuers had to navigate muddy terrain to reach the dolphins, which were scattered over a wide area along the east coast.

“It was a grueling 12-hour operation in the relentless sun, but the team was able to overcome the various challenges and give the dolphins their best chance of survival,” Misty Niemeyer, the fund’s stranding coordinator, told the broadcaster.

Rescuers went into shallow water to lure the dolphins away from shore. Once they were in deeper water, boats equipped with underwater ping devices – which make noise underwater – were deployed to herd the animals further out into deeper water.