I've taken a lot of photos of underground explorations, but regret not having taken more of the openings. Here are a few...
Previously posted, this is from Shasta Caverns. Now unused, it was formerly an access point before a larger entrance was created with dynamite.
A small cave on Maui - the entrance was so overwhelming fetid that I didn't venture in for fear of being overcome by the gases. It probably smelled something like the caves in our games though - stinking of orc, blood, and gods know what else. Not a lot of perspective here - it's about 5' high.
I've posted another shot of this one before, the roof collapsed in this tunnel in a cave in northern California. A tree fell in and would be climbable, although there are other easier and larger openings to the cave. In this shot, you can clearly see the two passageways diverging to the left and right. The other photo is here for perspective.
About 15 or 20' into another entrance of the previous cave. Many of the larger stones have been moved to the side at the front.