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Yes - radon, the silent killer. As well as Carrock/Force Crag and a large number of Cornish mines, other areas that ought to be treated with caution (especially with regard to unventilated headings) include the NE Wales limestone-hosted Pb-Zn mines, the SW Shropshire baryte/copper mines and the Ag-Cu mines of the Ochil Hills in Central Scotland. In each of the latter three groups there are occurrences of mineral pitch - hydrocarbon - in which microscopic inclusions of uraninite are present. Their radioactive decay leads to radon production. The Derbyshire mines, or those where "pitch" is found, are probably suspect too, but I'm less familiar with them. John