Last week thousands of people squeezed their way into Yosemite National Park, and you can bet that nearly all of them spent some time gaping up at Half Dome, the park's crowning attraction. And yet not a single one of them noticed when a 5 million pound slab slid off the face of this rock and came crashing to the valley floor.

It wasn't until July 5th that a pair of climbers over halfway up the 2,000 foot cliff face noticed that there was a massive section of missing rock preventing them from reaching the next place to anchor their rope. "There was a big dirt outline where the ledge was supposed to be," wrote Dave Miller on the climbing website SuperTopo, relaying the information from his perpelexed friend to a message board. Yes, it's weird that nobody noticed a 5 million pound slab of rock slide off the face of the most iconic attraction in the one of the most popular national parks in the country. But it turns out huge slabs of rock fall off mountains in Yosemite all the time.

Yosemite Valley—the main drag of the national park—sees about 60 rock falls a year. "That equates to about one a week," says Greg Stock, Yosemite's chief geologist. Most of these occur due to a process called exfoliation, a result of the unique geologic processes that formed the valley.

About 25 million years ago, the North American plate started overriding the Pacific plate. As the leading edge of the Pacific plate was pushed into the hot mantle below, it melted, sending bubbles of magma up through layers of continental crust. Half Dome, El Capitan, and many of Yosemite's other peaks began as these bubbles, called intrusions. The bubbles didn't actually reach the surface. But over time, they were exposed as ice age after ice age sent glacier after glacier carving through the nascent Sierra Nevadas. Softer rock cut away, leaving the tough, granite intrusions sticking out—Half Dome among the largest and most striking.

The last glaciers retreated about 15,000 years ago. Since then, the dominant process shaping the granite extrusions of Yosemite Valley has been exfoliation. This is exactly what it sounds like: thin flakes of rock sloughing away. "Rock will expand or contract as it heats and cools," says Stock, Yosemite's chief geologist. That means it swells out when the sun is up, and shrinks in the shade or nighttime. But the heating is never even—because the sun hits different parts of the dome at different times. Or not at all. The inner layers of the Half Dome don't get nearly as hot, so the expanding outer sections start to form inner cracks that run parallel with the dome's face.

But what makes the cracks snap is a mystery. In this case, Stock guesses that rain might be the culprit. "Water can get in the cracks, build pressure and push outward," he says. The Half Dome slab—measuring around 200 feet tall by 100 feet wide and 3 to ten feet thick—probably fell off sometime in the late night between between July 2 and 3, when there was heavy rainfall in the park.

So what happens next? "Climbers are always at their own risk in Yosemite," says Stock, though he adds that the community will probably take a break from Half Dome until they can fully assess the stability of the remaining rock by measuring the widths of visible cracks, or using acoustic sensors to listen for subsurface fissures. But the cliff is over 1,000 feet tall, so that's easier said than done. "We've never been able to document anything that's a clear precursor to imminent rock fall," says Stock. Occasionally, a gunshot-like cracking sound precedes a collapse, but that's not usually the case.

But in the longer term, the Regular Northwest Face—which was the first established route up Half Dome's sheer cliff face—will probably be OK. "In some ways, the route is almost more simple now," says Stock, who is also a climber. "That section used to involve moving over a bunch of broken rock and getting into this crevice so you could squeeze up. But all that rock is gone now and it's just a blank face." A few new bolts, and everything should be fine.

There's no guarantee that Half Dome won't exfoliate again. If it does, the best anyone can hope for is that it happens during those rare hours when nobody is gaping at—or climbing up—the sheer cliff wall.