About 60 dolphins swam onto the shore of Cape Cod, south of Boston, in Massachusetts last week, baffling marine life experts by their huge numbers.

The animals have been swimming onto the shores since Thursday last and about 60 of them have been stranded all along the shores of Cape Cod Bay so far.

It was one of the largest such cases in years, the International Fund for Animal Welfare (IFAW) said in a statement.

The IFAW has managed to release a few dolphins back into the sea.

Many were already dead, and some were logistically impossible to reach, but we were able to respond to 13 live animals, and 11 were healthy enough to release...unfortunately one died and the other was euthanized due to injury, said Katie Moore of Cape Cod Stranding Network (CCSN), IFAW's marine mammal rescue and research wing.

The animal welfare group has started collecting data from the dead dolphins to know the reason for such mass stranding that happen in Cape Cod almost every year during winters.

One partial cause for the mass stranding is linked to the dolphins' tendency to move in a group. If one swims towards shallow water, many others may follow, the IFAW said.

Two common dolphins are prepared to be released by a team from the International Fund for Animal Welfare into Cape Cod Bay in Herring Cove Beach, in this Jan. 16, 2012 handout photo. REUTERS/M.Booth

A female dolphin and her calf are transported by a team from the International Fund for Animal Welfare and the New England Aquarium before being released back into Cape Cod Bay, south of Boston in this Jan. 14, 2012 photo. REUTERS/Julia Cumes

Director of the International Fund for Animal Welfare's Emeregecy Relief Program, Ian Robinson, watches as a common dolphin swims off after being released back into Cape Cod Bay in Sagamore Beach in this Jan. 14, 2012 photo. REUTERS/M.Booth

A dolphin is transported by a team from the International Fund for Animal Welfare and the New England Aquarium before being released back into Cape Cod Bay, south of Boston in this Jan. 16, 2012. REUTERS/M.Booth