The carcass of a whale shark    Plos One

A deep-sea "graveyard" of giant sea mammals has been discovered by chance off the coast of Angola.

Amazing footage has emerged of a whale shark carcass and three deceased rays located in a small area on the seabed.

Scientists say to find four carcasses in such close proximity was "unprecedented" and must be connected to large food-falls in the area.

In five decades of deep-sea photography and exploration only nine carcasses have ever been documented, they said.

The chance discovery was captured by engineers from BP Angola, who were surveying the seafloor for industrial exploration.

The cause of the death of the animals is unknown but "ship strikes" or "accidental entanglement" could be blamed.

The findings have been published in the journal Plos One in which scientists also discuss the associated fauna and the role of large food-falls on deep sea ecosystems.

Carcass of a Mobulid rayPlos One

Lead author Dr Nick Higgs, from the University of Plymouth's Marine Institute, said: "There's been lots of research on whale-falls, but we've never really found any of these other large marine animals on the sea bed."

Researchers found scavenging fish (up to 50 individuals per carcass), mostly from the family Zoarcidae, feeding on or around the remains.

"We found three to four different types - but what really dominated were eel pouts. These normally sit around the carcass and wait for smaller scavengers - amphipods - to come along, and they will eat them," added Higgs.

The dead creatures were found between 2008 and 2010 on a one-square-kilometre patch of the sea floor.