A chunk of rock "the size of an apartment building" fell off the granite face of El Capitan in Yosemite National Park, killing one person and injuring another on the mountain at the height of climbing season, officials and witnesses said.
At least 30 climbers were on the wall at the time, but it was not clear if the victims were climbers or tourists, ranger Scott Gediman said.
"It was witnessed by a lot of people," he added.
The injured person was taken to a hospital near the park.
El Capitan is one of the world's largest granite monoliths, towering 4,000ft (1,219m) above Yosemite Valley.
Several people made emergency calls, reporting the rockfall from the Waterfall route on the east buttress of El Capitan.
Officials did not provide details about the size of the rockfall, but climbers posted pictures on social media from hundreds of feet up the wall showing billowing white dust moments after the crash.
"I saw a piece of rock - white granite the size of an apartment building, at least 100ft by 100ft - suddenly just come peeling off the wall with no warning," said Canadian climber Peter Zabrok, 57, who was scaling El Capitan and was above the rockfall.
Amazing shots of the rockslide at El Capitan in Yosemite Nat. Park. The photographer said it sounded like a huge explosion. pic.twitter.com/3yzQ4XS5jt— Bailey Miller (@BMillerFOX10) September 28, 2017
Mr Zabrok said he has climbed El Capitan dozens of times and has "never seen anything like this".
Mountaineers from around the world travel to the park in the Sierra Nevada to scale El Capitan's sheer face. Fall is one of the peak seasons because the days are long and the weather is warm.
Rockfalls are common in Yosemite but seldom fatal.
Ken Yager, president and founder of the Yosemite Climbing Association, reviewed photos of the cliff face and debris field, estimating that the relatively thin piece which broke off covered an area big enough to fit five houses.
"It cratered and sent stuff mushrooming out in all directions," said Mr Yager, fearing that the victim was someone he knew from the climbing community.
Mr Zabrok said he and friends were in the middle of a six-day climb of the Waterfall route on the right side of El Capitan when they saw it.
Rock slide in Yosemite's El Capitan kills one, injures another, photo courtesy: Dave Miller.. pic.twitter.com/M86f87fA3c— Felix Cortez (@FelixKSBW) September 28, 2017
"Boy, I don't know how anybody could have survived that," he told KFSN-TV, the ABC affiliate in Fresno.
Peering down from his perch 2,000ft (609m) up on the rock, Mr Zabrok said he saw a rescuer lowered by helicopter and "I believe he grabbed one survivor".
He later saw rescuers moving someone on a litter.
"It was done at tremendous peril to the rescuers because there were three subsequent rockfalls that were all nearly as big and would have killed anybody at the base," he said.
Climber Kevin Jorgeson said he and climbing partner Tommy Caldwell witnessed a massive rockfall in the same area while they prepared for a trek that made them the first people to free-climb the Dawn Wall on El Capitan in 2015.
First they heard a rumble and then they saw a white cloud of dust.
"Yosemite is just a really active, wild place. It's always changing," Mr Jorgeson said. "It doesn't make it any less tragic when someone gets in the way of that."
In 2013, a rock dislodged and severed the rope of a Montana climber who was scaling El Capitan.
Mason Robison, 38, fell about 230ft (70m) to his death. It was his gear digging into the side of the mountain that caused the rock to dislodge.
Yosemite remained open after Wednesday's rockfall, and other activities throughout the park were not affected, rangers said.