Assuming that air can equalise and the bottom of the mine is 2.5km below sea level, from air pressure calculations air pressure would be about ⅓ more, 1⅓ atm.

Assuming that air can equalise and the bottom of the mine is 2.5km below sea level, from air pressure calculations air pressure would be about ⅓ more, 1⅓ atm. I had an idea to see if plants would grow faster in a denser atmosphere. I’ve found one small study, I think the air pressure was thought to be denser when dinosaurs were about.