El Capitan, on the north side of Yosemite Valley, is a 3,593-foot wall of granite, challenging for even the world's best able-bodied climbers.
Try scaling it without the use of your legs. That's what Maj. Phil Packer of the British Royal Military Police accomplished early Thursday morning.
"Keep thinking every pull up is a message of support to anyone with disability to try a sport and big wall climbing as one of them," Packer wrote on Twitter, midway through the climb.
Rewind to Feb. 19, 2008.
Packer, 38, was serving as a military police officer with his unit in Basra, Iraq, when a rocket attack left him paralyzed below the waist. He also suffered damage to his upper torso and he was told by doctors that he'd never walk again.
"I think I thought I lost my legs. I looked down and it just felt as if they weren't really there," he later told a film crew during an interview.
About a year after his injury, Packer participated in the London Marathon, using crutches to complete the 26.2-mile trek two weeks after it began.
Since the paralyzing attack, he's also rowed across the English Channel and skydived with the Red Devils.
It's all part of his attempt to raise $1.9 million for Help for Heroes, a charity for disabled veterans. He's become something of a celebrity in his country, and this week's El Capitan climb helped him exceed his goal.
"I've set out to get a million pounds, and I got it," Packer said in the video interview. "Selfishly, this is something I'm doing to get through my life, and the year, by doing these different challenges.
"I don't do heights. I don't like them at all. It's a personal challenge, a personal battle, but if I can enthuse young people to do this kind of thing, one person would be enough for me."
Most people finish the climb up "El Cap" in three to five days, said Yosemite spokeswoman Kari Cobb. The fact that Packer finished in four, using only his arms to pull him up, makes it all the more impressive.
The route they took was a 1,800-feet ascent.
"It's definitely one of the harder routes," Cobb said. "The more weight and pressure you put on your legs, and the less you can use your arms, the better."
Packer didn't have that option. He told the United Kingdom's Daily Telegraph newspaper that he would be using his arms to do the equivalent of about 4,200 pull-ups, with the assistance of a specially rigged paragliding chair to pull himself up the rock formation.
"I actually pull three meters of rope for every meter I climb, which means that I actually pull myself up El Capitan three times," Packer wrote on Twitter.
He was assisted in the climb by veteran mountain climbers Andy Kirkpatrick, Ian Parnell and Paul Tattersall.
Kirkpatrick, the expedition leader, has scaled El Capitan's face more than 10 times.
"It's one of the most intimidating things you could attempt," Kirkpatrick said in a YouTube video made prior to the trip. "When you see it, the scale of it is just massive. Most people would say, 'There's no way I can do it.' Every time he moves one inch, he's raising some money, and that's a huge incentive. He wouldn't do it for fun, because it would just be cruel and unusual punishment."
Throughout the week, Packer and the crew updated Twitter and his personal blog site -- www.philpacker.com -- and fed YouTube with video clips of the trip.
To help Packer around the large obstacles, the climbing team strategically hung a series of ropes, swinging him to the next position.
They battled wind, fatigue and dehydration.
At 1,200 feet up, Packer said his arms were getting sore. To sleep and rest, he sat on a "portaledge," a collapsible device made of canvas and PVC pipes and suspended by ropes.
Early in the trip, Packer and his team noticed that the kit they brought along was more cumbersome than anticipated.
To ease the burden, they ate the "heavier" food items and drank as much water as possible.
El Capitan is a popular challenge for rock climbers around the world.
Cobb said that a "couple hundred" climbers attempt to scale its granite face each year, but the National Park Service doesn't keep records because climbing doesn't require a permit.
Packer isn't the first disabled climber to take on El Capitan -- Mark Wellman did it in 1999 -- but Packer's climb has attracted the attention of parkgoers and worldwide media, Cobb said.
Indeed, Packer reflected on Twitter how "surreal" it was to take media interviews halfway up the monolith, but he said he was grateful for the attention.
"Delighted the U.S. media are taking an interest in what we are aiming for," he wrote.
Upon reaching El Capitan's peak early Thursday morning, Packer told his 862 Twitter followers that he was "feeling very tired but emotional at the thought of what we have achieved, looking forward to a hot bath."
A few minutes later: "I think I have cured my phobia of heights."