I'm trying to mount a TV in my basement. House was built in the 60's, and the basement used to be a bomb shelter, from what I've been told, but most of the cinderblock walls were taken out. I'm trying to mount my TV on the wall, and when I drill in, I drill through the drywall, before hitting a hard spot. With a wood drill bit, I can apply extra force, and it will force in, but soon after, I hit a hollow area. I can't get lag bolts to tighten, as they start to go in somewhat tight but eventually I start feeling a grinding sensation and they end up getting loose before they are fully in.
Am I drilling into old Cinder block? It doesn't feel like the hard area I'm drilling in is very thick... maybe 1/2-3/4 of an inch, which seems too thin to be a cinder block. TV is ~70 pounds, am I safe to mount it there? Should I use anchors? One side of the TV will be mounted into a standard wood stud, so will that likely provide enough support?