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Indeed Gary, and East Kent has known reserves of coal that are deeper than those of Sheffield. Plenty of these reserves are under the seabed and along Kent's coast, in particular in the Dover District area. This fact was established towards the end of the 19th century.
However, I can't see it being possible to extract coal without leaving heaps of debris about in the vicinity of the shaft entrances. Even if this coal-dust were to be replaced in the shafts once exploited, this would not come about before the same shafts were no longer in use.
So this could mean after many months or even years.
My point is, would it still be feasible to develop land in the Dover area for large-scale building, such as at Whitfield, if the same area became designated for coal mining? I doubt it. And let's be clear on this, no matter which technology one uses to mine coal, it cannot be done without leaving heaps of underground debris on the surface.
And no mining of coal beneath the seabed could be carried out without an entrance from the shore. This entrance could not be at sea-level, as underground water, and sea water, would flood the shafts. So the entrances would have to be at a higher level, such as, for example, at Whitfield.