More than 8,000 firefighters are battling flames in northern California as wildfires that have killed dozens of people continue to rage.
The fires are the deadliest in the state's famous wine country since 1933 and officials are warning it could get worse.
How do wildfires start?
A fire needs three things: fuel, oxygen and heat. Firefighters often talk about the fire triangle when they're trying to put out a blaze.
On a hot summer's day when drought conditions peak, something as small as a spark from a train wheel can ignite a raging wildfire. Sometimes, fires occur naturally, ignited by heat from the sun or a lightning strike.
However, most wildfires are because of human carelessness such as arson, campfires, discarding lit cigarettes, not burning debris properly, playing with matches or fireworks.
It is not yet clear what started the fires in California, but officials say power lines blown over by strong winds could be the cause.
Why can they spread so quickly?
Once it's started, a wildfire can spread due to the wind, being on a slope or because of fuel.
"Where a slope is going upwards at a 10% gradient that would double the speed of the fire, if it's 20% it would quadruple the speed of the fire," Rob Gazzard, technical advisor to the Forestry Commission tells Newsbeat.
"That's because it's pre-heating the fuel above it. So if a fire is going up a mountain it will go very fast."
Fuel includes everything from trees, underbrush and dry grassy fields to homes. The more fuel there is, the more violently the fire will burn.
Plus if it's really dry - like it has been in California - the fire it creates is much more difficult to control.
What's the weather got it do with it
Quite a lot actually.
Low rainfall causing a drought, searing hot temperatures and wind all make the perfect recipe for a wildfire.
Remember the heat in the fire triangle? The sticks, trees and underbrush on the ground get hot from the sun, then very dry.
As it gets even hotter those "fuels" can ignite and that's why wildfires tend to rage in the afternoon, when temperatures are at their highest.
How to 'win' the fire
It's all about removing the fuel because you can't control the weather.
Fire experts use wildfire prediction tools which look at the wind speeds, slopes, the fire's direction and fuel to work out how to put out the blaze.
"You go hours or days ahead of the fire and remove anything that can fuel it using bulldozers, tree harvesters, hand tools," says Rob.
"You then create a fuel break [basically a big trench] around the whole of the fire. Then you can win and suppress it really effectively."
It can happen in the UK too
We tend to get them in England and Wales, as well as part of Northern Ireland and Scotland near urban and rural areas. Fires here are generally smaller because our temperatures aren't as extreme as Australia, New Zealand, South Africa, Canada or United States.
"According to the government's climate change risk assessment wildfires are predicted to increase in the United Kingdom by 30-50% by 2080," says Rob.
There is even a practice guide on preventing wildfires in the UK.
This involves picking less flammable trees, planting them in places where they're less likely to set on fire and harvesting them regularly.
Find us on Instagram at BBCNewsbeat and follow us on Snapchat, search for bbc_newsbeat