In February 1981, a 12-year-old boy named Todd Domboski was walking through his grandmother's back garden in Centralia, Pennsylvania, when the ground opened beneath his feet.

A hole about a metre wide had appeared, and it was deep and full of steam. Todd fell in. He grabbed onto the roots of a tree and clung on, shouting, until his cousin heard him and pulled him out. Afterwards, the air in the hole was tested. It contained dangerous levels of carbon monoxide.

The ground in Centralia had been hollowed out by a fire that had been burning underneath the town for almost twenty years.

Coal country

Centralia sat in the anthracite coal region of eastern Pennsylvania, where the ground is laced with coal seams and old mine tunnels. For more than a century, coal had made the town. In its heyday it had over a thousand residents, churches, shops, hotels and bars.

In 1962, the town council arranged to clean up a rubbish dump in an abandoned strip-mine pit at the edge of town, ahead of Memorial Day. The usual method was to burn it.

The most widely accepted explanation for what happened next is that the fire found its way into an exposed seam of coal at the bottom of the pit — and from there, into the network of old mine tunnels beneath the town.

A fire nobody could catch

Underground coal fires are notoriously hard to put out. They burn slowly, with limited oxygen, and spread along seams and through old tunnels in ways that are hard to map. Several attempts were made to excavate the fire or cut it off with trenches and fill. They were too small, too slow, or ran out of money. The fire moved on.

By the late 1970s, residents were noticing strange things. Basements became warm. Steam rose from cracks in the ground in winter. Gas station owners found the petrol in their underground tanks was getting hot.

After Todd Domboski's fall made national news, the pressure grew. In 1984, the US Congress set aside money to buy out residents and help them move. Most accepted. Houses were demolished one by one.

The town that wouldn't quite die

A small number of people refused to leave. In 1992, the state of Pennsylvania used eminent domain to take ownership of the remaining properties. Legal battles followed, and eventually the last few residents were allowed to stay for the rest of their lives. In 2002, the US Postal Service withdrew Centralia's ZIP code.

Today, Centralia is mostly fields and empty streets laid out in a grid, with trees growing where houses once stood. A handful of homes and a church on the hill remain. Steam still rises from the ground in places.

Life above the fire

For the people who stayed in the 1970s and early 1980s, the fire was not an abstraction. Some families had carbon monoxide detectors installed in their homes by the state, and readings were checked regularly. Trees in some areas died as the ground beneath them heated up. On cold mornings, plumes of steam drifted from vents and cracks, carrying a sulphurous smell.

The town was divided over what to do. Some residents wanted to fight the fire and save Centralia; others wanted to be bought out and leave. The arguments split neighbours, churches and families.

How long will it burn?

Nobody knows. There is enough coal beneath the area for the fire to continue for a very long time — estimates of a century or more are often quoted. It is one of thousands of coal fires burning underground around the world, from China to India to the United States, which together release large amounts of carbon dioxide and other pollutants.