You're not thinking of trying to explore it are you? I wouldn't. Extremely low oxygen and non-existent airflow with all kinds of nasty stuff in the atmosphere.
I know a group of lads who explored some of it about 10 years ago but it was with full BA kit, Atex lamps and gas detectors.
I don't know much about the mine, other than much of it is under a military base of some sorts.
As Miles says, its not nice.
Buckhill can probably give you a lot of interesting history to the place and it is of minor historical note.
The colliery itself is nasty, no oxygen (if I recall it is under 7% oxygen, which will kill you) and very low roof height.
For anyone thinking of having a poke underground, save yourself the bother. The risk and difficulty involved are far greater than the reward.
Even with BA kit you can't get far in. You run out of air before you can get very deep in and movement is difficult because of low roof height. By the time you've negotiated the first couple of shale collapses you have used half of your air tank. You'll spend the rest of your time knocking chunks of roof onto your head with the tank.
Its an interesting place, but not interesting enough to die for 😉