In A Walk in the Woods, the city of Centralia—a once-bustling suburb that's become a literal hellhole—represents the horrors of environmental pollution.
Here's the deal: Centralia was built above a large coal mine that provided jobs to many of the city's residents. Things went well for a while, but took a disastrous turn when a fire spread to the mines below the city, lighting a blaze that still burns to this day. For real. Check out pictures and videos from Centralia to get a better idea of how crazy this place has become.
Given Bryson's interest in environmental issues, he sees this city's sad fate as a warning for the future. Something like this could easily happen on a larger scale–for example, Bryson specifically states the global warming could cause "the whole of the Appalachian wilderness below New England" to "become savanna" (1.1.5). That's bad. If nothing else, we should look at Centralia as a cautionary tale for our treatment of the environment.