In Yosemite Valley, rockfalls can happen every four to five days, where boulders that can be larger than your average car or apartment building thunder down steep mountainsides.

Greg Stock is a geomorphologist who works alongside USGS engineer Brian Collins. It’s their job to monitor rockfalls in the park — which is critical, when 5 million visitors pass through Yosemite each year.

During a particularly memorable rockfall in 1996, for instance, a chunk of rock free-fell 3,000 feet to the valley floor, generating category five hurricane strength winds when it hit, blowing over 1,000 trees in a matter of seconds. So what happened in those final moments to cause the rock to leave the cliff?

There are probably decades or even centuries of preparing that rock to fall, but the 'trigger' is defined as that last element, the straw that breaks the camel's back. Roots and ice are known triggers, but roughly a third of the rockfalls in Yosemite happen where Greg and Brian aren’t able to identify a trigger.

Using a combination of high-res photography, LiDAR, aerial surveillance, and 3D modeling, the team is able to track and learn from events that may happen when no one is around. They also identify hazard areas where rockfalls have happened before. Though the team can flag unstable areas, it is difficult to predict when a rockfall will happen. Varying weather conditions, quakes and even wildlife can be the difference between a crack remaining stable for 100 years and a major rockfall event.

Using specialized gear call crack meters, the team measured how much a crack would open over the course of a day over three and a half years, and found something rock climbers and geologists had hypothesized but never proved — that rock falls could be triggered from hot weather.

It's Greg and Brian's data that helps to keep millions of Yosemite visitors safe each year.