My guess it is full of bad air. You might go in for 10 feet before you die. Let us know how you get on.
Eastern United Drift Mine. NGR SO 648 113 Ruspidge, near Cinderford. Opened in 1909 by Henry Crawshay & Co Ltd to work Coleford High Delf, c4 feet thick, by means of three, unique to the Forest of Dean, descending inclines. Nearby, Walmer's Shaft was reopened for ventilation / escape route. The workings abutted Cannop Colliery about 3 miles away and were 1400 feet deep. This mine was nearly abandoned during early development work as the coal dipped suddenly down vertically due to a monocline. The two adjacent drifts pushed on in front and completely missed the coal. The colliery was only saved by pushing the dipples sideways from the original headings and following the coal downwards until it levelled off in the bottom of the coal basin.
Eastern United cont
It closed with millions of tons of coal left in the vertical area because it was uneconomical to work vertically bedded coal. Closed by the National Coal Board in 1959. The pit head baths, tips, embankment and sidings survive.
References: Anstis 1999 p 16, 59, 71, 77, 82, 89, 90, 99. Beech p 24; Bowen 1991 p 17 - 27; Mullin p 87 Phelps 1984 p 41 Waters p 172
Cutting coal in my spare time.