Sadly this is nothing to do with crypto mining. This is something that was done years ago by coal miners.
I walk everyday and part of my walk takes me through a local wood. In this wood are giant hills and valley's. I often wondered what these strange contours were and after some research i found they were old bell mine workings.
A bell mine is a vertical mine shaft that is dug down till you reach the coal seam. Then the shaft is dug outwards with picks and shovels to mine the coal. These bell mines were dangerous places to work due to collapse
The hills on the site are spoils from the shaft and the deep holes are where the mines have collapsed over the years.
There is no way i would go down one of those shafts as a job let alone dig the base out for coal.
The history of these mines is quite fascinating although I'm glad to say the practice of bell mining is long gone
diagram source (http://www.staffspasttrack.org.uk)