A massive wildfire near Yosemite National Park is threatening to disrupt San Francisco’s water and power supply and to damage two groves of giant sequoias, Fox News reports. The cause of the fire is not yet known, but it started before the lightening storms that sparked numerous smaller fires across the state.

Sequoias San Francisco

The Rim Fire in San Francisco

The Rim Fire, as it’s being called, has forced authorities to mobilize more than a dozen helicopters and half as many fixed wing tankers to drop water and other fire suppressants to aid the 2,800 firefighters working on the ground. “This fire has continued to pose every challenge that there can be [in] a fire: inaccessible terrain, strong winds, dry conditions,” said Daniel Berlant of the California Department of Forestry and Fire Protection.

The immediate threat is to three towns north of the fire: Tuolumne City, Ponderosa Hills, and Twain Hart. Winds are driving the fire at speeds up to 50 miles per hour, and there are concerns that it could jump containment lines and threaten thousands of structures. The fire has already burned down the Berkeley Tuolomne Family Campsite, though authorities were able to evacuate the camp and prevent any injuries.

Fire moving towards Hetch Hetchy Reservoir

The Rim Fire is also moving towards the Hetch Hetchy Reservoir, which supplies most of San Francisco’s drinking water and power to some important buildings including the airport. Two hydro-electric stations have been preemptively shutdown, but the city’s water supply has so far been unaffected.

Giant sequoia trees

While not as important as protecting towns, two giant sequoia groves could be affected by the fire. Giant sequoias are naturally fire retardant, with bark that can be three feet thick at the base of a mature tree, but they are certainly not immune to a fire of this size and intensity. Giant sequoias are the largest and oldest trees on the planet—the oldest living giant sequoia is a tad younger than the Giza pyramids, so losing one to a forest fire is truly irreparable.

“All of the plants and trees in Yosemite are important, but the giant sequoias are incredibly important both for what they are and as symbols of the National Park System,” said Yosemite Park spokesperson Scott Gediman.