Bruce Stover, chief geologist for Colorado's department of natural resources, thinks about this fire all the time. Finding new ways to control coal-mine fires is his job.

Coal-mine fires are a burning environmental concern in Pennsylvania, Kentucky, West Virginia, Colorado and other mining states. The ground over these fires is weak and hazardous. Poison gases from such a fire can leak through to the surface, making the people who live nearby very sick. In 1962 toxic fumes from a fire below Centralia, Pennsylvania, forced all its citizens to move away. Right now, the town of Percy, Pennsylvania, is threatened by a long-burning mine fire.

Coal mines were built with wooden beams to prop up their tunnels. Thick walls of coal held the beams in place. An electrical storm, extreme heat or even an aboveground fire near an open coal-mine vent can spark a fire down below. When the wooden frame of a mine catches fire, the surrounding coal provides enough fuel to keep the fire burning for years. The fires are a waste of a valuable natural resource, coal, as well as a health and safety threat.

Where's the Fire?
It's nearly impossible to fight a fire that you can't see, so Stover's first task is to find the hot spot. "It's like playing the game Battleship," he says. "We can't really see what we're after, so we drill holes in specific areas based on old mine maps." Fire fighters can also spot mine-fire locations from the air. The fires' poison gases kill trees and grass above, leaving bare patches over the blazes.

Stover drops a gas-detecting meter down each hole he drills. If carbon monoxide, sulfur dioxide and methane gases are present, there's a fire burning nearby. He also drops in a thermometer to check the underground temperature.

Water cannot be used to stop mine fires. "Water boils suddenly, and with nowhere to go, an explosion would occur," Stover explains. Lately he has been testing something new: a special foam that looks like shaving cream and contains cement, ash, chemicals and air bubbles. Stover pumps it into the holes he drills. It seals out the oxygen the fire needs to grow. The bubbly foam seems to resist the heat very well.

The best that Stover can hope for is to keep the fires small and reduce the harm they do to the environment. He knows he's fighting a battle he'll never completely win: "We can't put the fires out."