Experts have been looking into the possibility of methane leaks in the town dump, unauthorized mining activities, and explosives usage, and I guess even fracking. But they haven't figured it out.
I hope it turns out to be nothing (or something to which it can be clearly attributed and then stopped), but for now the possibilities are — if media have taught us anything — as ominous as they are endless.
Off the top of my head, I'm imagining the scenarios to 2012, Tremors (great film, by the way), or Gasland (the fracking* documentary), or a South Park episode on lactose intolerance.
* Autocorrect is apparently in the pocket of Big Gas, because it keeps changing fracking to tracking.