Filming the BBC's landmark series Asia took its crew on a four-year-long odyssey from the open ocean to the "roof of the world".

From frozen mountains to parched deserts, and lush tropical rainforest to vast grassland steppes – Asia is Earth's largest continent and home to an incredible array of environments.

Perhaps because of that sheer size and variety, until last year the BBC had never devoted a wildlife series entirely to it. The vastness, the crowded megacities and the extreme diversity of environments makes it harder to encapsule in a handful of episodes.

The Natural History Unit's landmark series Asia took four years to make. "Many parts of Asia are extremely remote, largely unknown, or frequently off-limits," producer Matthew Wright says. "Its wildlife is less well-studied than that of Africa and the Americas, so we had fewer leads to go on when we started our research.”

"We started by scouring scientific papers, books, websites and social media looking for stories. We spoke to colleagues, conservationists and tour guides too. Once running orders were drawn up, we spent two years and over 2,500 days filming," said Wright.

The team captured everything from reef sharks hunting bright Moorish idol fish in the Pacific Ocean to the world's largest goats living amid spectacular scenery in Pakistan. They tracked elephants through dense tea plantations in India and spied on huge monitor lizards living in the heart of one of Asia's biggest cities, Bangkok.

Here, we look at some of the behind-the-scenes images captured by the Asia crew during the series' epic production.

The crew had to work out how to film reef sharks chasing after Moorish idols, a brightly coloured fish species, in the Pacific Ocean. The sharks are fast moving and too dangerous to film at close quarters when in hunting mode.

What's more, the Moorish idols themselves were fast swimmers, clocking some 10mph (16km/h). Researcher Seth Daood says the team created a retractable pole that could be dipped into the water when needed and pulled up when the boat had to increase speed to catch up with the hunt.

Using the pole alongside drones and other camera crew swimming close to the boat, the team captured an hour of footage as the sharks reduced the shoal of hundreds of fish to almost nothing.

The Asia team travelled to Qinghai province, in the west of China, to shoot the enigmatic fox on highland meadows nearly 4.5km (14,800ft) high. The crew needed three days to acclimatise to the altitude and head out to find a female fox that had been filmed by the team before.

The fox had learned not to see the team as a threat, says researcher Joshua Chen. "There were times when we were sitting on the grass, and she came around, and steadied herself into her sleeping position. She slept with her back to us, leaving us with the sense that she trusted us completely," he says.

The fox had given birth to a litter of cubs and spent much of the time hunting pika, small rodents, on the pasture. She used a variety of techniques, including using the bodies of lumbering yaks as cover, and the crew witnessed many successful hunts.

In another high-up pocket of the Asian continent, the team returned time and time again to capture not hunting wildlife but a meteorological oddity.

The........

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