A strong and fast-moving wildfire broke out in Mariposa County, California this weekend, burning 6,550 acres and threatening Yosemite National Park.

The Oak Fire started on Friday evening in the town of Midpines in Mariposa County and destroyed 10 structures overnight. The sprawling bush fire is now threatening 2,000 more structures, including some of the world’s oldest giant sequoia trees.

The fire was zero percent contained as of Saturday morning, spreading as much as 10.2 square miles and prompting an immediate evacaution of 6,000 people. 400 firefighters and multiple water-hose helicopters are working to contain the flames.

Oak Fire sparks massive Pyrocumulus cloud

The heat of the fires generated a spectacular and scary Pyrocumulus cloud on Friday evening that could be seen from the Bay Area.

There’s still no word yet on what sparked the fire in what could be an ominous warning signaling the start of a more destructive wildfire season.