The coal runs through the ground in vanes. Much of this area was strip mined, where they basically strip the surface a little at a time and collect the coal as they go. Eventually you end up with pits, sort of like you see in a quarry.
Well, the town of Centralia used one of the old pits as a landfill, and when the pit would get full, the local FD would burn the trash. Apparently the pit wasn't completely void of coal, and when they burned the trash, it set fire to the vane of coal. Once that vane gets set, it's like watching a slow fuse burning. They will just keep going slow and steady until they run out of fuel, oxygen, or get drowned in water. The problem with putting them out, is the vanes go hundreds, even thousands of feet in the ground, and can span miles in length, so getting to where the fires burn is nearly impossible.
The locals didn't take immediate action, and never really thought of it as a problem when it started. I guess they thought it would go out on it's own when they got a good rainfall or something. That never happened, and the fires started tracking under the town, and close to the surface where the houses were. I think the turning point when they realized they had a real problem was a few years later. The local gas station owner/operated stuck a stick in an underground tank to check fuel level, and when he pulled it out, noticed it felt really warm, when normally it would be ground temperature, about 50 degrees. He dropped a thermometer down, and the temperature of the fuel was something like 200 degrees, due to fires burning near the tanks. By this point, the problem had gone beyond the point of correcting it easily.
Eventually the gases and smoke started rising out of the ground, through the storm drains, basements, and everywhere else. Basements and foundations started caving in as the fires burned under the houses, toxic gases rose up through the basements, and one by one many of the residents were forced out of their homes. The government had thrown millions at trying to put the fires out, but in the end, nothing worked and they decided it would be cheaper to relocate the people than continue efforts to stop the fire. by the mid 80's the population was around 100 people, and the few residents that remained stayed there because they didn't want to give up their homes, regardless of the risks.
Sometime around 2005 the postal service revoked the zip code, officially making it a ghost town. I think at the last census, there were 11 people claiming Centralia as residence, and one of the houses has cleared out since then, so I'd say probably 7 or 8 people live there now.
You can still go to the town and see all the streets and driveways. If you didn't know what you were looking for, you could drive straight through the town square, and never realize that a thriving town with a population of over 3,000 had once been there. The sidewalks are overgrown with weeds, most of the roads are overgrown, and most of the structures have been demolished and the foundations filled in. 3 cemeteries stand throughout the town, but no churches to be seen. Just a couple houses still stand, and the municipal building which contains the fire department and ambulance are still there and being maintaned by one of the die hard residents.