> I think the point is you can't go far with those fuel sources (e.g. you can't have a trucking industry that runs on whale oil).
Yeah, that's true but you can have electric trains and trams. Canals and sails are also totally useful for a coastal society.
> How deep? In a thousand years (let alone a million), will the mining tunnels have collapsed and/or filled with water? Even if the tunnels still exist and are serviceable, an "energy poor society" is probably not going to pump out the many millions of gallons of water needed to get at the coal.
In the US alone, they're all over the place. There's 250 billion short tons of coal that's estimated to be commercially recoverable in the US today. A good bit of that isn't particularly deep. There's still surface coal all over West Virginia. A lot of of it just gets left around because its too dirty to burn under current law, or is a little to disperse to mine profitably in current energy markets.
Pumping isn't that hard if labor is cheap, the Romans did it with two cylinder pumps, person powered water wheels, and screw pumps. Pumping water is actually a key challenge that leads to important technology, or it did for us.