This fire in Yosemite National Park will consume 70 acres of forest—as big as 68 football fields. And, it was deliberately started by man.

For most of the last century, naturally occurring fires in Yosemite, usually caused by lightning, were put out whenever they started.

The result was an incredible build-up of deadwood and undergrowth, which fuelled catastrophic infernos.

Now national park firefighters play catch-up. They manage and allow naturally occurring fires to burn or set controlled fires like this.

TARO PUSINA, YOSEMITE FIRE SPECIALIST: “We can’t let all fires burn and we can’t put all fires out. We have to find that medium, but understand that fire is a vital and important part of our ecosystem”

This fire has special significance, and special risks, because it is burning in one of the most historically important few square miles of all Yosemite.

The Mariposa Grove of giant sequoias- Yosemite’s largest remaining stand of these great trees.

The sequoias are an iconic symbol of Yosemite. This grove was one of the main reasons President Lincoln took time out from the Civil War to declare Yosemite a protected place in 1864.

When it was policy to stop fires at all costs, few sequoias sprouted.

Why--- was an enduring mystery finally solved just a few decades ago.

Giant sequoias depend on fire to reproduce.

The heat opens their seed cones, their seeds are released, the flames clear the earth for their germination.

While lesser trees blaze around them, the giant sequoias stand virtually unscathed by the flames.

They’re remarkably fire resistant. Their bark is possibly the thickest of any known tree on earth, sometimes more than two feet, at the base of the tree.

It prevents flames of fast burning fires from reaching the interior of the tree.

TARO PUSINA, YOSEMITE FIRE SPECIALIST: "They've seen fire for eons and they will continue to see it if we have anything to do about it."

When the flames die down, the earth is ready for a new generation of giant sequoias.

They’re one of the oldest living things on earth. Some have been around some 3,000 years. And they stand as tall as a 27-story building.

One of the natural symbols that make Yosemite National Park one of America’s greatest wonders.