Why did the muck fires constantly burn in Tangerine?

The muck fires constantly burn because of lignite.
Wayne explains that lignite is basically a step before coal. It's a substance that feeds the fires so that they never stop. There are hundreds of miles of lignite under the fields. He says that even when it rains, the muck fires are still burning. They might burn a little less, but they never completely go out. When the winds are high, he tells them, people shut their windows and stay inside.
The muck fires cause Paul and his family's new home to have a terrible smell that won't go away. Wayne says that if you want to stop them entirely, you'd have to stop lightning from hitting the fields in the summer. So far, he says, they haven't figured out a way to control the lightning.
Later, Mr. Costello says that they hired contractors to sink four wells into the muck fires. It didn't stop the fires from burning. However, it did create a swamp that brought lots of mosquitos, which are now creating new health issues for the community.