I am not sure if this is mentioned before, but I didn't come across while reading this forum
When you tear down a building you get some resources back (I think it's 25% ?) But the interesting part is that it doesn't seem to matter how much remains in the deposit. So when you tear down a coalmine just before depletion you still get 50 HW 50M & 37T back! (the same as a mine you just build)
That's 1 mine for free on every 4 mines! Just keep a good eye on the remaining amount and bomb the mine just before it reaches zero.