The world population of saiga antelope halved in less than two weeks after 140,000 animals perished on the Kazakhstan steppes without any obvious cause.
Shocked conservationists found entire herds dead or dying with saiga littered over the vast open grasslands, mostly mothers or newborn calves.
The only explanation being given so far is that usually harmless bacteria may have wiped out the herds.
Tissue samples retrieved from the dead animals showed toxins, produced by Pasteurella and possibly Clostridia bacteria, which are believed to have caused extensive bleeding in most of the saigas' organs before they died.
Mass deaths are not unknown among grazing herds when they gather in large numbers at nursery sites, which makes them more prone to outbreaks of bacterial infections such as pasteurellosis, but this scale is unheard of, prompting the more bizarre speculation.
Christiantoday.com reported the event under the headline: "Apocalyptic sign? 60,000 antelopes in Kazakhstan mysteriously die in four days."
A number of conspiracy theory websites then began asking if the deaths could be connected to the Blood Moon.
Among the even more bizarre theories was the involvement of aliens.
Scott C Waring, who edits UFO Sightsing Daily, posted: "This Antelope mass death was an experiment by aliens. It is well known to the Kazakhstan Government that aliens are making use of an underwater base in the Caspian Sea which Kazakhstan boarders."
The Head of Azerbaijan's national Aerospace Agency has confirmed this allegation when he stated bluntly that the old USSR constantly monitored alien spaceships that were frequently recorded entering the water, but due to security concerns, kept this a secret. "
There has not been a ground swell of support for this theory, however.
Saiga, with their strange elephant like probosces used to filter dust, have already been all but wiped off the map by poachers keen to cash in on their valuable horns which sell at £2 a gram on the Far East medicine markets.
The horns are powdered down and used to make a cooling drink said to bring down fevers.
For the saiga, persecution by hunters has seen their global population reduced by 95 per cent in less than 25 years, a catastrophic decline accelerated by the fall of the Soviet Union and the opening up of previously closed borders.
This decline has now reached crisis point after the sudden and inexplicable mass die off this summer.
UK wildlife charity, People’s Trust for Endangered Species, has launched an urgent fund-raising appeal to help scientists investigate the reason for the tragedy. It is backing the Saiga Conservation Alliance’s on-the-ground efforts to unravel the mystery.
But scientists have been left baffled by the scale of the mortality, with herds miles apart succumbing at the same time.
Nida Al Fulaij, grants manager at PTES, said: “This event is simply catastrophic for the long term survival of this critically endangered species. Right now we need the public’s help, and donations are urgently required.
"This was an abnormal occurrence, and it’s important to us to find some immediate answers.”