This summer I visited the remarkable Slanic mine in Romania [1]. No matter how much you prepare, it's still breathtaking when you get there. The chambers I visited were all about 50 meters tall. You could have football fields in there, bleachers included. I took pictures, but I just don't know how to share them; still, I found this one [2] that gives a sense of the scale.

[1] https://en.wikipedia.org/wiki/Sl%C4%83nic_mine

[2] https://www.romania-insider.com/planetarium-slanic-salt-mine