Climbing the slick side of a frozen mountain requires nerves as cool as the ice under your crampons.
But what happens when the ice can't be trusted any more? It's a question renowned ice climber Will Gadd has had to face.
The way he remembers it, conditions were near perfect: an August day along a route climbed many times before. He and his group were following a well-trodden path to a well-known mountain hut built hundreds of years before.
Still, it was an area where he had to watch out for rockfall. Just in case.
"I discussed with my guests what our plan would be. If rocks were falling, I would say left or right to get them to move," says Gadd.
"And as we are going across the slope, I hear the unmistakable sound of rockfall."
But, as Gadd paused and looked around, there was nothing. No movement at all. So he and his group continued as fast as they could to get to the safer ground.
And then suddenly, it happened. Loose rocks tumbling down, heading straight for the climbers.
"Move right!" yelled Gadd. His group scrambled to dodge debris slicing though the thin air.
They were lucky. The group escaped with one broken arm between them. They were shaken, but Gadd knows it could have been worse.
"We had to wait for rescue to show up," Gadd says.
"During that time, no other rocks in the whole valley came down, nothing. The specialist came in and was just standing there. I told him to get behind the rock because there's gonna be more rockfall out there.
"He said, 'Nothing's moving, you're just being paranoid'. I said 'Maybe, but look, we've got an issue here."
So what caused this unexpected rockfall?
"I suspect those rocks and that accident had to do with melting permafrost."
Having travelled all over the world to climb some of the toughest ice around - from the Alps to the frozen winter waterfalls of Niagara - there are few ice climbers more experienced than Canadian Gadd.
Scaling a rock face alone would be an adrenaline rush for most. Climbing up frozen water on the side of said rock face? Even more so. What about paragliding off the top of that rock face, too?
"I'm not normal," he jokes.
But for Gadd it's about more than chasing a thrill.
"I don't do these sports because they're hazardous. Like, if I wanted just danger I could go run back and forth on the highway dodging cars. There's no interest in that, whereas flying a paraglider over the Grand Canyon, or climbing a big frozen waterfall is intensely interesting. It's complicated physically and intellectually and that's what fires me up."
Now things are changing. Ice climbing is becoming even more challenging, for reasons beyond his control.
In 2014, Gadd took an expedition to Kilimanjaro in Tanzania, east Africa, the tallest freestanding mountain in the world. Kilimanjaro is known for its ice cap and glaciers and has been made famous in the climbing world by figures such as the legendary Reinhold Messner.
It was in fact the pioneering route taken by Italian Messner in 1978 that drove Gadd to return to Kilimanjaro in 2020.
Gadd had noticed Kilimanjaro's receding glaciers during his first visit, comparing reality on the mountain with the maps and photographs he had for navigation. On his second trip, the picture was even starker.
"I wanted to shoot the same locations that we'd shot five years earlier and see what had changed," he says.
"But there was no ice. You know, big chunks of things were gone. And the little pieces that I shot before were totally gone."
This melting of ice is one of the most visible symptoms of climate change. Kilimanjaro
Whether it be glaciers, ice sheets, sea ice or permafrost, rising temperatures are causing ice in all its forms around the world to melt.
In turn, and in combination with other factors, this contributes to increasingly extreme climate conditions around the globe. Melting ice caps and glaciers add to increases in sea level, and more moisture in the air causes instability in weather patterns.
Ice also plays an important role in mitigating the effects of global warming. It reflects back the sun's light and heat, while sea ice traps heat within the ocean, preventing it warming the atmosphere above. As ice melts, those checks on rising temperature disappear with it.
For Gadd, his climbs are becoming more difficult and dangerous as the permafrost that acts as the glue holding rocks together disappears. He has been playing an important role in documenting how his sport has changed.
In 2018, Gadd was named a for his work helping climate scientists in their research.
That same year he was one of the first people to climb underneath Greenland's ice sheets.
Gadd's expedition was documented in and showed how his experience of ice climbing was able to help scientists make new discoveries about climate change.
"If these places are really hazardous, it's not a place I'm going to run towards now," he says. "But for these researchers it was trying to figure out how these systems in Greenland work. We all read about the Greenland ice sheets melting.
"It's pretty nice to make a difference in a small way."
Gadd describes himself as a "canary in the coalmine" when it comes to the effects of climate change.
In the documentary 'The Last Ascent', which followed Gadd's trip back to Kilimanjaro in 2020, the emotion of seeing just how quickly the ice had disappeared is plain to see.
"I'm an ice climber and when I show up there, my ice climbs are gone," Gadd says.
"On Kilimanjaro, the climb I'd gone to do was gone. I have pictures showing that for the previous 50 years, every year, that climb is there. I knew it would change, but I didn't think it would change that much.
"I hope people can look at that and go 'wow, what's going on?' Hopefully it resonates a bit more than just a dry scientific paper.
"The world can live without ice climbers and ice climbing. But those things disappearing is just a symptom of what's going on."
Speaking now, about 10 years on from that near miss on that August day in the Canadian Rockies, much has changed in his life and work since.
Previously he would fly an estimated 100,000 miles a year - "I was a complete carbon criminal" - travelling around the world as a professional athlete, competing at events such as the X Games.
Now aged 55, he says: "When you almost get killed by falling rocks, it does tend to alter your perspective."
It's led to several lifestyle decisions, including eating fewer animal products and finding more energy efficient ways to heat his house. Most significantly he has reduced his carbon footprint in travel.
While he has made personal changes - "we don't need to be perfect, we just need to do better" - it's particularly important for Gadd that he uses his platform to help inform others.
"I was talking to people on a tour and they were interested: 'What's the crazy climber doing?'" he says.
"And that's a really great discussion, but then about half of them were there as ice tourists. They wanted to come and do this walk before it was gone.
"I'm seeing more and more of that, where people are coming to Canada and the glaciated regions to see the ice.
"Before it is gone."