/diy/ - Do It Yourself

/diy/, what the fuck are my walls made of?

Just bought a house, doing some initial work on it installing some ethernet outlets, and I discovered that my walls are not made out of drywall, or anything I've ever seen or that my friends have ever seen. This shit is hard as rock, that middle layer looks and feels like concrete, and the rear layer is even harder. Drill bits squeal as I drill into the material it's that hard.

House was built in the 1950s. I don't think the walls have been redone since then, though I'm not certain. Additionally, there's zero insulation in the outer walls -- but the house is actually super cozy and comfy, so I'm honestly wondering if this isn't some sort of insulating material. Insulated cementboard is apparently a thing, but the pictures I find online don't look like this. Also, there's definitely no horizontal component here, I've cut out a 1ft section and it's all just exactly like pic related, so I don't think it's any form of lath and plaster.