Please don't enter coal mines without the necessary gas detecting equipment, you are unlikely to get out alive!--
Sorry but to be MORE explicit!
If it's Coal stay out! There are too many hazards that make it deadly.
Anyway most coal is well sealed and the coal authority take a VERY poof view of anyone breaking in.
Try to find a local caving (and mine exploring ) club who will have knowledge and equipment to help you. (Plus most of them like drinking beer and dressing up in funny clothes!);(