Presume there is a coal mine. Presume it has been there for a hundred years and presume that it has provided jobs for people who can reasonably get to a coal mine in a reasonable amount of time.

That coal mine is a local resource that can be exploited on the global marketplace. It exchanges a local good - coal - for global resources. You can't eat coal, but you can trade it for food. More generally, coal can be sold to provide for the livelihood of everyone who mines it.

The people who mine it need things other than coal. As a result, ancillary businesses develop surrounding the coal mine - department stores, grocery stores, etc. While the local economy is diversified into most business segments, the economic driver remains coal.

Now suppose the world needs less coal.

The first thing that happens is that there is less value in what comes out of the ground because scarcity has diminished. Companies that own coal mines start doing things like selling them. The people who work at that coal mine will continue to be employed so long as someone buys the coal mine and pays them the same wages, which rarely happens. There will also be layoffs as less coal mined means less people to mine it.

The next thing that happens is that those coal miners buy less stuff. Your ancillary markets suffer - the grocery stores, the car dealerships, etc. Now people who do not mine coal are suffering because of coal prices.

But you can move away, right? Except the only asset most families own is their house... and a house that was purchased in a thriving economy is worth a whole lot less than a house in a failing economy. So you're going to take a muther of a hit just moving away (fun fact: my move last year cost me $9k for a family of three from one metropolis to another) and then another muther of a hit divesting yourself of your house and then you're going to need to find another job and if you're in coal... well, you're fucked. And if you're in sales or other ancillary business... well, there are no markets that suddenly need a whole bunch of ancillary support except other boom industries, like fracking, which is also getting hammered.

So what you're left with is a bunch of people who used to work in a thriving industry in a town that used to be thriving who can't afford to go anywhere else because the thriving was a long damn time ago.