More than 350 Botswana elephants have suddenly died near watering holes, a phenomenon conservationists are unable to explain.
Conservationists have seen more than 350 elephants dead in the Okavango Delta in the northern part of the country since early May, according to Mark Hiley, co-founder of UK-based charity National Park Rescue.
"As far as the scale is concerned, it's the biggest thing that's happened to elephants this century," he said.
Elephants of all ages and sexes are dying, and roughly 70% of them were found nearing watering holes.
Cyanide, anthrax, and drought are all being considered, but conservationists said all three were unlikely.
"Some of the elephants are running around in circles, others dragging their back legs, suggesting the potential for toxins affecting brain function," Hiley said, adding that an unknown pathogen is most likely the cause.
The Botswana government has not yet run tests.
"A professional team needs to go in fast and the government are not allowing that," Hiley said. "They're turning down offers of support, which is very unfortunate. And there's no doubt that the lack of cooperation is causing more elephant deaths. It's a tragedy given that they're one of the most persecuted species on the planet."