Good lord man! That's never a good sight to see.
Wasn't my house, a neighbors. He was overseas and his housekeeper was staying at the house and using the fireplace. The owner was too cheap to get the flue cleaned out unless he started to lose draft (huge flue). Even then he told the sweep not to bother with the debris in the cleanout. The door for the cleanout is outside and you can see it in some of the pictures. The cleanout box is huge ~ 3 X 4 X 4'.
Anyway here is the story.......I was getting ready to turn in about 11 PM when the housekeeper knocks on the door a little hysterical. So much she couldn't remember the address! I called the fire dept and went over to check things. There was lots of smoke inside the house but no real flames coming from the chimney, yet. The FD arrived with a small truck and a pumper truck, lots of firemen in personal vehicles. They hosed down the roof and went topside to inspect the chimney (see picture), it had a stainless cap.
So they opened up the clean out door. All there was to see was creosote. They didn't have a shovel!!! Two FD trucks and no shovel.........I found a few in the barn. They started to shovel it out and eventually reached some burning creosote (they burnt up 2-3 shovels).
A little later the fun began as they removed enough creosote so the air was able to go up the flue. There is a picture I posted with the liquid flaming creosote coming out of the chimney cap. At this point the pumper truck is keeping the roof wet, and a couple firemen went up on the roof and knocked the cap off. That's when the fire sounded like a jet engine with ~10-15 feet of flame coming out of the chimney. Globs of burning creosote raining down, would have lost the house without the pumper truck. The heat was intense enough the crack the exterior of the large brick chimney, but no flames from those or in the house. After a while it burnt itself out.
A few more pictures. The 2nd one is when they just started on the cleanout. The last one is one of the burnt shovels on top of the creosote pile.