Underground City or the Child of the Cavern, The by VERNE, Jules

Covering a time span of over ten years, this novel follows the fortunes of the mining community of Aberfoyle near Stirling, Scotland. Receiving a letter from an old colleague, mining engineer James Starr sets off for the old Aberfoyle mine, thought to have been mined out ten years earlier. Starr finds mine overman Simon Ford and his family living in a cottage deep inside the mine; he is astonished to find that Ford has made a discovery of the presence of a large vein of coal. Accompanying Simon Ford are his wife, Madge, and adult son, Harry. (Summary by Wikipedia)