Caves under mountains and mines which utilise them have existed as long as mankind has been capable of exploring them.
Tolkien was mining a rich seam there (ouch, sorry).
Although he didn't grow up in a mining community, he was very much aware of it. He used to watch the coal trucks going to and from Wales. It's referenced in Genesis, U A Fanthorpe's poem about him (posted somewhere on the Cellar)
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