A wildfire near Yosemite National Park has exploded into one of the largest wildfires in California history putting crucial utilities for the city of San Francisco in peril.

This wildfire, known as the Rim Fire, started on August 17th in a remote canyon in the Stanislaw National Forest and is still just 5% contained.

Extra dry grass and brush have helped to feed the fire that has now consumed over 125,000 acres of Northern California.

Steep terrain, warm weather and low humidity have hampered efforts to contain the blaze. So-called “crown fires” or a fire that burns the tops of trees have been blamed for the fire jumping at least two “firebreaks” firefighters built to contain the blaze.

More than 2,600 firefighters spent Saturday with bulldozers and water-dropping aircraft battling the blaze near the Hetch Hetchy water and power system which supplies the San Francisco metro area with its basic utilities.

The San Francisco Public Utilities Commission has already been forced to shut down two of its three hydroelectric power stations. Officials have been able to purchase power on the open market and disruptions have been at a minimum. Water from nearby reservoirs that supply San Francisco have not been contaminated.

California governor Jerry Brown issued a state of emergency for the area earlier this week but expanded the order to the San Francisco area because of the threat to the city’s utilities.

Nearly 5,000 homes are threatened by the fire and the vacation communities of Groveland and Pine Mountain are under voluntary evacuation orders.

Rangers in the Yosemite National Park are taking precautions to protect three dozen of the park’s iconic giant sequoia trees. Giant sequoias, one of the largest and oldest living things on earth, naturally grow only on the western portions of the Sierra Nevada Mountains.

A volunteer sheriff from the Tuolumne County Sheriff Department was quoted in the Los Angeles Times saying this “fire was “coming in at a close second” to a rash of wildfires that scorched hundreds of thousands of acres throughout the state in 1987.”