The California Rim Fire is slowly but surely being contained by a team of more than 3,700 firefighters.

According to the California Department of Forestry and Fire Protection (CAL FIRE), the Rim Fire now stands at 20 percent containment, up 5 percent from Monday morning. On Sunday, the fire was only 7 percent contained, showing that the 3,752 firefighters on duty are slowly gaining control of the situation. Still, more than 4,500 structures are considered threatened by the Rim Fire, and at least 23 have already been destroyed.

By late Monday night, the Rim Fire had grown to 160,980 acres (roughly 250 square miles), but its rate of expansion is slowing. The fire grew 16,000 acres over the past 24 hours, only half as fast as the fire grew daily last week, the Mercury News reports. However, while the fire may be slowing down, it is still the largest active fire in the United States and the 13th-largest fire ever recorded in the state of California.

As the fire continues to grow, it poses a major threat to the San Francisco Bay Area, located roughly 150 miles away. The Rim Fire is progressing towards Yosemite’s Hetch Hetchy reservoir, the Bay Area’s primary water supply. Gov. Jerry Brown has already declared a state of emergency over the fire’s threat to water reservoirs.

The Associated Press reported on Monday morning that the fire was approximately five miles away from the reservoir and already begun to send ash into the reservoir. However, the ash has not yet reached intake pumps and is reportedly non-toxic.

Throughout Monday, crews rushed to move water from Hetch Hetchy into other local reservoirs in the event that Rim Fire ash does eventually contaminate the reservoir. Public Utilities Commission General Manager Harlan Kelly Jr. said that a recent expensive upgrade to the city’s piping system had also helped to expedite the process.

Kelly said that San Francisco currently has a six-month back up water supply. Brown’s emergency declaration on Thursday, which referenced “conditions of extreme peril,” also permitted the city to spend $600,000 on replacement electricity, Bloomberg reports.