When a fire looks like it’s going out, it may be about to get worse.

One of a firefighter’s biggest fears is something called “backdraft.” To put it simply, this is when the fire in a building has consumed most of the available oxygen, causing it to look like it’s subsided.

In reality, the place is now filled with heat and flammable gases, and when someone does something like, say, open a door or window, the mixture of the incoming oxygen can react violently, leading to occasionally explosive results.

Watch these firefighters show what this might look like here: