Tromso Skyrace is more than just extreme. When describing the course in 2014, race director Kilian Jornet admitted: "You could die."
It was no exaggeration.
At the halfway point of the 57km route comes the most challenging section: an exposed, steep, narrow ridge approaching the 1,404-metre Hamperokken summit.
During the 2017 race, American skyrunner Hillary Allen fell from that ridge. She was in freefall for 50ft. Then she tumbled another 100ft down the rock "like a rag doll" before crashing to a halt.
This is the story of how a 31-year-old woman from Colorado returned to run the race that almost killed her.
It was 5 August 2017. Allen was looking forward to a "fun day out" with no pressure. She remembers smiling, saying hello to friends and new faces along the course. One of those was a fellow competitor named Manu Par, a Spaniard who lives in Tromso.
Allen became a professional skyrunner in 2015 and spent each summer racing in Europe. By 2017 she was one of the leading athletes on the Migu Run Skyrunner World Series and chose to make Tromso her last race before heading home, where she's also a science teacher.
Located in the far north of Norway, where mountains rise sharply off the coast, the Tromso race has a special place in skyrunning. The truest form of the sport goes from sea to summit.
Its route takes runners along trails, through forests, across snow and boulder fields, and up to the area's most iconic summits - Tromsdalstinden (1,238m) and Hamperokken (1,404m) - for a total elevation gain of 4,800m.
Allen passed Manu Par at the start of Hamperokken's 3.5km ridge. She was in her element, picking the right line across the rugged terrain, making steady progress. Then disaster struck.
Par was 10 metres behind when Allen fell. It was almost a sheer vertical drop and he saw her bounce down the mountain, screaming as bits of rock broke loose and fell with her. It seemed to last as long as 10 seconds.
"The worst thing was the sound," says Par, 31. "A human body bouncing against the rock. It was just awful."
Instinct took over. Par put his own safety at risk by scrambling down the rock to reach Allen. What he found was a crumbled heap. Her body was twisted, her arms were like bags of bones, there was a gash on her thigh so big that Par could have put his hand in.
"I was sure she was dead," he says. "I didn't even think to check her vitals."
But after a few seconds he realised her stomach was moving. She was still breathing. Adrenaline kicked in. Par is trained as a mountain guide and swiftly called on the basic first aid he knows.
Allen was in danger of falling further so first he had to move her, but not too much as it was clear she had a spinal injury. She regained consciousness and Par told her not to move, urging her to stay awake.
"You could see she was fighting to stay alive, to do what I told her," he says. "It was incredible. Just imagine being in that situation - most normal people would have given up."
Some race photographers also witnessed the fall and called for help. A rescue helicopter arrived after around 25 minutes. Allen's precarious position meant it took two hours to hoist her safely from the mountain.
Remarkably, Allen survived. She had 12 broken bones, including two in her back and both arms, and needed hundreds of stitches. Over the next two weeks she had five operations and was told she would probably never run again.
But within a year she was back competing in skyrunning. Soon after she decided that she would return to Norway. She needed closure.
Allen can't recall exactly what happened - whether she slipped, tripped, or a rock broke away from underfoot. But she does remember falling.
"Time slowed down," she says. "I remember the impact of hitting the ground but I don't remember the pain of it. I remember the sensation of my bones breaking, the sound of it.
"I was thinking: 'This is it, you're going to die.' I remember relaxing, even though it was a pretty terrifying moment, and thinking: 'Do your best to stop yourself, but just embrace it.'
"I passed out and when I came to I saw Manu and the other people rescuing me. When I saw their faces I thought I was going to die. I'd never seen that look of terror before. Then the pain hit. It came in waves."
It was so intense it caused her to scream, until the pain relief took effect, and then she was airlifted to hospital. Par visited Allen the next day.
"There were so many tubes and she was completely groggy from the anaesthetics," he says. "I still thought she was going to die until two days after."
It was only when Allen woke that day that the severity of her injuries dawned on her too.
"I couldn't move, there were wires coming out of me, stitches and cuts everywhere," she says. "I thought 'oh my God, can I even function again?' Never mind run."
As well as breaking both arms and two vertebrae, she had broken several ribs and bones in her feet. She suffered a lisfranc fracture in her right foot, and it was that which jeopardised her ability to run again. It required screws which were later removed, although the plates in her arms remain.
The first time Allen posted on social media after the accident was three days later - an Instagram video from her hospital bed in which, still drowsy from the pain relief, she slurs her words while listing her injuries.
Back in Colorado a week later, she posted another video in which she becomes tearful while describing the operations she's about to have.
"I didn't look pretty," she laughs now. "When I watch them back, I grimace. But I don't care because that's where I was at.
"That was a pact I made early in my recovery. I have mixed emotions about social media. I feel like a lot of the time it's this big lie. You never see the real struggle, the raw emotions.
"I wanted to be honest about what happened. Initially, it was about showing family and friends that I was OK, but from there on out I received incredible support via social media.
"I continued to publish the good and bad moments, to document how extremely hard the recovery process was and continued to be."
Allen returned home with only one limb that "kind of worked". Every little thing became a huge task - sleeping, eating, washing, dressing. She couldn't shower or go to the bathroom unsupervised.
"Some days I didn't have the energy to get out of bed. Early on I wished that the accident killed me because it would have been easier."
Gradually, she found ways to cope. She made a contraption to eat with and now laughs about the number of people she hardly knew that saw her naked.
She couldn't use crutches so one of her sponsors provided a bespoke scooter on which she could bear weight through her elbows. Naturally, she broke it going "off road" in parks and along trails and had to get it fixed at a bike shop.
Within three months she could walk again, within six she could run, then after 10 she entered her first skyrace since the accident - on 17 June 2018. The week after that she did the 48km Cortina Trail race in the Dolomites in northern Italy - and won it.
The idea of returning to Norway had always been at the back of her mind. By early 2019 she was planning to race again in Tromso that August.
During a routine training run in February, she broke an ankle. But she recovered in time to win the Cortina Trail again in June. Tromso was back on.
"As I crossed the line at the Cortina Trail I was like: 'OK, I have to go back. It scares me, and it's hard, but I need to go back'," says Allen. "I felt ready to face the fear."
Par agreed to race with her. They had kept in touch but when Allen returned to Norway it was the first time they'd seen each other since she left Tromso. Three days before the race, they went back up to the ridge and the very spot where Allen almost died.
"It was kind of weird," says Par. "We had a really close connection through what happened but didn't really know each other. That was the first time we ever talked properly."
Allen wanted to know everything about 'that day'. How Par found her and what he saw. They had never talked about the accident in detail before - and they haven't since.
Par says: "It was like a run plus therapy, it was just something we had to do."
Allen adds: "I knew the accident was bad but hearing it from Manu's perspective was pretty intense. For the rest of the day I just didn't want to be around anyone. I was actually contemplating whether to stay for the race because I didn't want to go back there. It made me realise how lucky I am to be alive. It was cathartic."
Allen had "the most fun" as she and Par completed the race together, talking and laughing, even on the ridge.
"There was no doubt in my mind that I was going to finish," she says. "It was a weight that I had on me for two years. Now I feel free, liberated. I don't hold a grudge against the mountain anymore. I spent so long being afraid of that place but now I see it for its pure beauty."
A self-confessed science nerd, Allen was studying for a Masters degree in neuroscience and playing competitive tennis but sought a "more simple release". She tried trail running in 2013 and "things just clicked". She felt it was what she was meant to do. After her fall, she didn't know if she would ever recover to be an elite athlete again. But without it, who was she?
During her recovery she spoke to a sports psychologist, who helped her develop a sense of self worth that did not depend on competition. She now feels the ordeal gave her the opportunity to rediscover why she loves running and has made her a better athlete - as well as a better person.
She's found a new sport (gravel riding), is trying different types of training and running further than she's run before. In August she came second in one of the Ultra Trail du Mont Blanc races, the 145km Traces des Ducs de Savoie.
"It's shown me what I'm capable of from this new perspective of 'I don't care if I win'," she says.
"It's given me more perspective, more depth. I've got more freedom to discover what works for me, how far I can push myself, to learn more about myself - and I wouldn't trade that for anything.
"People call me brave. I don't necessarily think that. Yeah, I'm stubborn. I like doing hard things, facing my fears and finding a way through, finding solutions in situations that seem impossible.
"Hopefully that's what I'm now defined by - my character and integrity. Life is hard and if I can help others confront the challenges they face then that surpasses anything I achieve in running."