On the morning of Feb. 7, 2017, two electricians were working on a warning siren near the spillway of Oroville Dam, 60 miles north of Sacramento, when they heard an explosion. As they watched, a giant plume of water rose over their heads, and chunks of concrete began flying down the hillside toward the Feather River. The dam’s spillway, a concrete channel capable of moving millions of gallons of water out of the reservoir in seconds, was disintegrating in front of them. If it had to be taken out of service, a serious rainstorm, like the one that had been falling on Northern California for days, could cause the dam — the tallest in the United States — to fail. Kory Honea, the sheriff of Butte County, which includes the dam and the town it is named for, first heard that something was wrong from Dino Corbin, a local radio personality, who called him at his office: “Are you aware there’s a hole in the spillway?” Around the same time, one of the sheriff’s dispatchers received a confusing message from California’s Department of Water Resources, which owns the dam, saying it was conducting a “routine inspection” after reports of an incident. At the dam, department officials closed the gates at the top of the spillway to prevent any more of its concrete slabs from being lost in what an independent forensic report prepared after the incident described as “a sudden, explosive failure.” The flow of water stopped. The rain, however, didn’t. In the six years since the near-failure of the Oroville Dam, dam operators across the country have begun to reassess the structures under their control, looking for hidden weaknesses: the cracks in the spillway, the hillside that crumbles at the first sign of water. That work is necessary, but it may not be enough to prevent the next disaster. Bigger storms are on the way.