Our Planet makes a point of saying what other nature series have not—the wonders they’re showing are endangered because of humans—and the footage is perhaps the most shocking part of a series full of discomfiting moments. Contrary to popular belief, not even lemmings dive off cliffs. Why would a walrus? Polar bears weren’t harassing them. The camera crews were filming from afar so their scents and sounds wouldn’t spook the skittish animals. Then why? What were walruses even doing on cliff tops in the first place? Our Planet offers a clear answer. “This is the sad reality of climate change,” Lanfear told me. “They’d be on the ice if they could.”

Read: Netflix’s Our Planet says what other nature series have omitted

In the summer, Pacific walruses forage for shellfish in the waters between Alaska and Russia, before hauling up onto sea ice to rest and raise their young. But in recent years, Arctic sea ice has been thinner and sparser. The 2017–18 season marked a record low. As these icy platforms have retreated, walruses have increasingly been forced to haul out onto solid land—in the thousands.

These haul-outs aren’t new events, but they were once rarer, smaller, and less dangerous, according to Anatoly Kochnev, a Russian naturalist who has studied walruses for 36 years. When he started, only males gathered on these sites; now females and calves do too, and many are trampled in the scrum. When he started, haul-outs were rare in the northerly Chukchi Sea; now many sites there regularly heave with walruses.

With Kochnev’s help, the seven-person Our Planet team filmed one of the largest haul-out sites—a single beach where 100,000 walruses tessellate into a solid red mat of tusks and blubber. The animals arrived almost overnight, while the team slept in a cramped hut. “It was like 100,000 Chewbaccas outside,” says Lanfear. “We could hear tusks scraping along the side of the walls. We could hear walruses snoring. We opened the door, and it was a wall of blubber.” The walruses gather “out of desperation, not out of choice,” David Attenborough says over the resulting footage. “A stampede can occur out of nowhere. Under these conditions, walruses are a danger to themselves.” And so they climb “to find space away from the crowds.”

As the walruses spread across the beach, some start heading up a shallow slope, which curves into a steeper escarpment, which eventually culminates in 260-feet cliffs. It’s not an easy climb, but Kochnev suspects that once one group leads the way, the rest follow their scent. And since this area gets very little rain, odor trails from previous years might lead new arrivals up a dangerous path. “At least up here, there’s space to rest,” Attenborough intones. “A walrus’s eyesight out of water is poor, but they can sense the others down below. As they get hungry, they need to return to the sea. In their desperation to do so, hundreds fall from heights they should never have scaled.”