OK, I am a little late, but I saw the prompt for this week's Sepia Saturday and had to join in.
Rocks! Overhanging rocks!
On the front: " The mysterious hanging boulder mystifies all who pass under. Polar Caves, N.H."
On the back: "Polar Caves, N.H., America's greatest Boulder Caverns. A maze of caves and spectacular rock formations. All caverns colorfully lighted. Different from and [sic] caverns you may have seen. Nothing like it on the American continent."
I love older tourist postcards like this.
Polar caves in Rumney NH is still there and still an active tourist location. Its website is, of course, http://www.polarcaves.com/. I think the hanging boulder is still there.