I started picking at a small corner of it, thinking I would just take down a small piece and see what's underneath. That's when we found this:
I couldn't stop there! Within about 30 minutes, we had the whole wall down. The wallpaper underneath is beautiful. If it was in better condition, we would actually consider keeping it.
The gypsum board that we removed is dated 1944. We figure the wallpaper looks like it's from the 20's, and was probably put up just before the Depression, then was covered over once the economy had recovered. Underneath this layer are two others, one that is VERY dark (probably from the turn of the century) and another lighter green floral that we think was most likely the first paper from when the house was built in 1883. It goes all the way down the wall, behind the wainscoting panels.
Tomorrow we plan on starting to drywall the room. We'll be using 1/4", the thinnest that you can buy. We don't need it to be structural, just a good clean surface to prime and paint or wallpaper over.
I'm certainly glad we found this though! It's like stepping back in time, which is what we were hoping to do as we go through the process of renovating this old house, which is finally starting to feel like our home.