A horrified TV crew working on Planet Earth II saw 150,000 antelope killed off by disease in just three days as they filmed at calving grounds.
Fans of the David Attenborough series will see scenes featuring the Saiga Antelope of Kazakhstan in Central Asia on Sunday.
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But Chadden Hunter, producer of the Grasslands episode, told how he thought he was witnessing the "extinction of the species" as the animals started to die.
He said: "When we were out there in the calving grounds, with hundreds of thousands of females all giving birth at the same time, a very virulent disease swept through the population and killed around 150,000 of them in a matter of three days.
"At the time we thought we were watching the greatest natural catastrophe that I'd ever heard of. We watched 150,000 of these magnificent animals die in front of us.
"At the time we didn't know if it was the final extinction of the species, which was devastating, emotionally, for the crew."
Filmmakers had driven "deep into the middle of Kazakhstan" for days searching for calving herds of the "amazing" creature.
He said: "To go somewhere so remote to see an animal that looks like it's from another planet was just incredible."
Despite the mass deaths, the Saiga Antelope has not been wiped out.
Chadden said: "We've since heard that the last few mothers and babies we filmed have survived. It was a potent reminder of how fragile yet resilient nature can be."
Planet Earth II has been a huge hit for BBC1 and on Sunday 9.7 million viewers tuned in to watch the desert episode, while over on ITV, ratings for The X Factor peaked at just 7.6 million.