It has been one of the most mesmerizing nature series to grace the small screen in recent years, with many pulse-racing chases, kills, interesting facts and variation of locations.

Whilst filming the episode due to be aired on Sunday (4 December) the crew captured the devastation of 150,000 antelope dying from a bacterial disease in just three days.

Fans of the David Attenborough-narrated series will witness the upsetting scenes featuring the Saiga Antelope of Kazakhstan in Central Asia.

In a shocking revelation, producer of the Grasslands episode, Chadden Hunter, explains how he believed he was witnessing the "extinction of the species" as the animals began to perish.

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."

The film-making team had searched "deep into the middle of Kazakhstan" in cars to find calving herds of the animal, but weren't expecting the horror they encountered.

He continued: "To go somewhere so remote to see an animal that looks like it's from another planet was just incredible."

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."

The mass die-off took place in May 2015. At the time, scientists estimated that almost half of the world's population of the saiga antelope had been killed by a mystery bacterial infection.

Despite the huge number of deaths, the Saiga Antelope has not been wiped out. They are a critically endangered antelope, however, that originally inhabited a vast area of the Eurasian steppe zone from the foothills of the Carpathian Mountains and Caucasus into Dzungaria and Mongolia.

Today, the dominant subspecies is only found in one location in Russia, and three areas in Kazakhstan. In spring this year, the Ministry of Agriculture of Kazakhstan carried out a population survey to reassess the numbers of saiga antelopes. In total, 108,300 adult saigas were observed, most of them in the Ural region.

Planet Earth II – which has been a huge hit for BBC1, garnering 9.7 million viewers on last week's episode – continues this Sunday at 8pm.