A mega-herd of a quarter of a million Mongolian gazelles has been seen gathering on the country's steppes, one of the world's last great wildernesses.
The coming together on the grassy plains is the largest ever recorded.
The biologists who saw it estimate it contained perhaps a quarter of all Mongolian gazelles on the planet.
"It was stunning," says Kirk Olson of the University of Massachusetts, US. "I don't know if I was surprised or simply blown away by what we came across."
Olson and colleagues based in the US and Mongolia have published details of the epic gathering in the journal Oryx.
In September 2007, Olson's team were driving across the eastern Mongolian steppes studying the habitat of the Mongolian gazelle, one of the last nomadic ungulates to survive in large numbers.
Together with scientists at the Smithsonian Institute, they had been capturing gazelles and fitting them with GPS collars to track their movements, trying to work out where they travel and why.
As they drove east they began to encounter herds of a couple of thousand individuals.
"Groups of this size are impressive and beautiful to see," describes Olson. Then the following day, at about midday, they drove to a hillside offering a great view of what appeared to be one such herd.
"But it was really one edge of a group that ended up being over 250,000 by one estimate.
"We were simply amazed at the sight. The image I have in my mind of seeing this massive aggregation of gazelles will always be etched into my memory."
Mongolian gazelles are known to gather in large herds. Groups containing 10,000 animals or more are often reported, while the largest herd previously known numbered 80,000.
"I expected that we would come across gazelles at times in large and impressive numbers," says Olson. "But not a couple hundred thousand in one sweep across the horizon. I had never seen that many before and that many had never been documented."
Olson believes the gathering was a natural event triggered by a set of rare and extreme circumstances.
The summer of 2007 was incredibly dry with some areas of the steppe experiencing a severe drought.
The gazelles were quickly running out of places to graze, and ended up concentrating in the few remaining green areas.
One in particular covered an area of undisturbed land uninhabited by people. A huge intense rainfall two weeks earlier had also watered its grass, making it an ideal refuge for the nomadic creatures.
Habitat fragmenting
"The fact that bit of suitable habitat exists and the gazelles were able to access this during an extremely dry summer is at present good news," says Olson. "It means the steppe is still intact."
However, he cautions that such huge herds may not survive.
Local people say that the animals used to herd in such numbers that the ground before them appeared to move. But the grassland steppes are increasingly being carved up by fences, roads, agriculture, and densely settled areas, while oil fields and pipelines are being developed in the region.
"The 250,000 sq km in Eastern Mongolia is simply the remaining natural example of a much larger ecosystem that spanned over Inner Mongolia and Manchuria totalling about 1.5 million sq km," he says.
If a similar set of circumstances were to happen again, but the only suitable grazing areas were just 30 km south, then a border fence would have prevented the animals reaching food.
"We would have been reporting a massive die-off of gazelles."
Hunting has already decimated populations of the saiga antelope and the kulan, also known as the Mongolian wild ass. Olson hopes that conservationists will increase their efforts to protect the gazelles, before their huge herds are also reduced.
"Grasslands and long distance migrants throughout the world are facing intense pressure and numerous threats," he says.
"Eastern Mongolia is arguably one of the best remaining examples of an ecologically complete grassland in the world. The Mongolian gazelles are the largest remaining population of wild ungulate remaining in Central Asia," he says.
"What will seal the fate for the gazelles is if their habitat is degraded and fragmented to a point where the large herds that exist today no longer have a place to go, and then they will be lost."