190,000 people were evacuated in Norther California after one of the tallest dams developed a hole that threatened to release uncontrolled floodwaters. The emergency slipway off the Oroville Dam was the second to fail in a matter of days, after the dam’s primary slipway developed a 200-foot-long, 30-foot-deep hole last week.

The dam itself was not damaged. But because its water levels are so high following heavy rain, if the emergency slipway collapses it could unleash a wall of water onto communities and rivers below.

People have started leaving the area, gassing up at stations and on route to evacuation zones. One resident Sean Dennis said: “We “We both were kind of shocked, nothing like this has ever happened,” said the 30-year-old chef from Yuba City, recalling the moment he and his wife found out they had to leave. “We just grabbed what we could.”

Officials ordered the evacuations on Sunday after they spotted the damage to the emergency spillway’s concrete lip. This auxiliary spillway was being used for the first time in almost 50 years, according to The Associated Press, because 770-foot-tall dam was full to the brim and its main spillway was damaged by heavy rain last week.

Over one million acre-feet of water could be released, flooding communities in Butte County, Yuba City and Marysville.