Editor's Note: Tens of thousands of firefighters are currently battling wildfires in the Western United States. This year, millions of acres have been lost to fires in California, Washington, Oregon, Idaho, Montana, and Alaska. Three firefighters died while on duty in Washington State Wednesday.

National Geographic photographer Mark Thiessen, who has training as a firefighter, traveled with a crew from Salt Lake City, Utah last week as they fought a group of blazes about 250 miles north of San Francisco. So far, more than 117,000 acres of California forests have burned (update: 144,000 have burned as of August 15).

HAYFORK, California—I’m on assignment, embedded at the Fork Complex fire in Northern California. I’ve shot wildfires for years and I want to bring you with me to an active fireline on an out of control forest fire. It’s a world inaccessible by most media. I went through fire school so I could have better access to the fire and bring you the story of how a wildfire is fought.

Step One: The Burnout Operation

View Images Division Supervisor Jay Walter starts a burnout deep in the forest. His modified starter pistol fires Quickfire Hotshot Flares (nicknamed "sausages") that turn into fireworks when they land. Photograph by Mark Thiessen, Nat Geo Image Collection

“Firing!” he yells as he aims between two ponderosa pines and pulls the trigger. An explosion of sparks shoots from his pistol and spirals like a roman candle 300 feet (91 meters) into the forest. Jay Walter, a division supervisor who is in charge of this section of fireline, reloads and fires off another volley of fireworks. His goal is to ignite an island of green vegetation. He’s fighting fire with fire. It’s the most effective way to control the blazes burning in the forest.

Firefighters don’t just spray water at wildfires, and that’s not because California is in a drought. In burnout operations, firefighters start their own fires to burn the plants that would otherwise get caught in a wildfire. These controlled burns start at the control line—a boundary such as a river, a road, or a strip cleared by a bulldozer. Low-intensity ground fires like this are actually good for the forest. They clean out the understory and leave more resources for larger trees.

View Images NG STAFF; AS OF AUG. 18

Prepping for the burn takes time. Earlier in the day, McKay DeGering, 21, a member of the Salt Lake Unified fire crew, worked as a sawyer—with his chainsaw, he thinned the woods near the control line to stop the burnout from climbing into the canopy. The sawyers tear through the forest like a machine. “It’s great when you have three saws all ripping together down the line,” DeGering says. “It’s when you get clicking you get your mojo going.”

Sawyers work in tight concert with partners called swampers, who throw aside the cut brush and limbs. Chainsaws are loud, so sawyers and swampers communicate with facial expressions and the occasional hand signal. “Everyone reads the saw,” DeGering says. “The saw sets the tempo the way a conductor directs an orchestra.” (Learn more about the firefighters in this story on our photo blog, Proof.)

Night is a good time to do firing operations like this, because the temperatures are cooler, the humidity is up, and it’s easier to monitor and keep the fire in control. Fires are hot, dusty, and smoky, but they can also be amazingly beautiful. The ground fire causes the rising smoke to glow orange, illuminating the forest from within. The forest looks magical at times like this and I’m reminded how beautiful fire can be.

WATCH: Sometimes the best way to fight a wildfire is with fire.

Step Two: Holding the Line

In my time shooting forest fires, there’s one thing that has become perfectly clear to me. Fire always finds a way. Forests like fire. It’s as if the forest has a ravenous appetite for fire because it knows it needs fire to stay healthy. Once the controlled burn is set, firefighters try and hold the line and stop the feeding frenzy.

Holding can be slow, boring work, but firefighters have to watch carefully for spot fires. Spots pop up across the line and risk taking a controlled burn out of control.

The drought in California has made holding harder. Forests are so dry that they’re burning hotter and more severely than before. Firefighters no longer have confidence that small control lines put in by hand can actually hold a fire; they now use wider lines built by a bulldozer.

“Holding line is the single most important thing we do out here and the least glamorous,” says Walter. “Letting the fire jump your line is the worst thing that can happen to a week’s worth of work. You work your ass off for days just to have it made obsolete in a few minutes”.

WATCH: On the front lines, the Salt Lake Unified fire crew is working to contain forest fires.

Step Three: Mopping Up

The hose sprays a precisely aimed jet of water deep into the smoldering stump, splattering dark, sooty water all over the firefighter. Welcome to mop up. It’s hard, dirty work, but it’s extremely necessary. This is when the fire actually goes out completely. By the time the crew is finished, the fire’s outer edge is cold to the touch.

Mop up can last for days after the last active flame along the control line has been extinguished. Stumps near the line can burn for a long time, and they have to be torn out and extinguished completely. Otherwise fire could smolder in the root system all winter and come to the surface on the green side of the control line in the spring, starting another fire.

The final step to mopping up is cold trailing. The firefighter removes his glove and feels the ground with his bare hand looking for heat. If he feels it, he sprays a little water on it and works it in with his shovel until it’s cold. Finally, this section of fire is completely out and tucked into bed.

I follow the Salt Lake Unified fire crew as they mop up, and the danger from fire may be less, but there are still risks. “Don’t go that way, take the long way around,” someone yells out to me, “There are bees over there.”

“Who has the most bee stings?” I yell back, not really expecting an answer.

“Me! 15!” replies Carlos Lopez. It turns out Lopez, a swamper, followed the lead saw through the underbrush. He was doing as all good swampers do by throwing aside the sawyer’s cuttings. This time the lead sawyer agitated a bee hive, and the bees took it out on Lopez.

This is what I love about wildfires. You never know what is going to happen next.