London - The mystery of a nine-million-year-old whale graveyard discovered in South America has finally been explained.

More than 40 fossilised whales were discovered in the Atacama Desert, Chile in 2010. The mass grave is believed to be the first example of repeated strandings of animals – but scientists didn’t know what caused them.

But experts now think the animals, which include baleen whales and sperm whales as well as seals and aquatic sloths, were killed by toxic algae.

Chilean and US palaeontologists found most of the whales at the Cerro Ballena site were facing in the same direction and upside down, suggesting they all died from the same cause.

The four different fossil levels showed it was not one but four separate events that happened over thousands of years.

Palaeontologist Nicholas Pyenson, who led the research, published in a Royal Society journal, said: “The key for us was its repetitive nature at Cerro Ballena.

“No other plausible explanation in the modern world would be recurring, except for toxic algae, which can recur if the conditions are right.” - Daily Mail