Nearly 3,700 firefighters working on the ground and from the air have fought the California wildfire near the Yosemite National Park, known as the Rim Fire, and established 20% containment, up from 15% at the beginning of the day.

The fire has burned through at least 251 square miles. At least 23 structures are destroyed and 4,500 more are threatened.

While the fire rages 20 miles away, Yosemite National Park remains open with tourists and campers visiting. Park ranger Scott Gediman said Ahwahnee Hotel and Curry Village located in Yosemite Valley, home to Half Dome, and almost all park areas are open.

“We’ve got clear blue skies here in Yosemite Valley,” he said, according to USA Today. “The park is busy. I was just out walking around, and there are plenty of people here.” In addition, the fire is burning on about 22,000 acres in the northwest portion of the park, “but that’s just 3% of the park as a whole,” he said.

Despite the park ranger’s optimism, firefighters, aided by 15 helicopters and 460 fire engines, are working hard to contain the fire as it is rated as having “extreme” growth potential.

The Rime fire is burning half a mile away from Hetch Hetchy reservoir, San Francisco’s main source of water, but contrary to past reports, the water supply is safe, said Charles Sheehan, a spokesman for the San Francisco Public Utilities Commission, according to USA Today.

“There’s no change to water quality,” Sheehan said. “We have instruments monitoring the water constantly.” Should the water turbidity changes significantly, the utility would switch to suppliers of water from nearby reservoirs.

Dry conditions are expected through Tuesday, but an isolated storm is possible, said Weather Channel meteorologist Chris Dolce. Although rainfall is beneficial, lightning and gusty winds may hinder containment of the fire.