The two friends - who don't want to be named - entered Middleton mine in the Derbyshire Dales, UK, after they spotted the "entrance was open".
300m underground - and 8km into the tunnels - they say they found a film set, including an American school bus, a stage, and a lift car, on October 18.
The pair claims "two people who helped support the staff and actors" said it was left behind after the filming of a soon-to-be-released Marvel movie, rumoured to be called The Fantastic Four: Blue Moon.
