You can walk right into the NE rock face near Novodmitrovsk at the northern base of the rock, near the woods.

I have marked the location on the image included.

After walking in it's possible to exit through the same spot, but once you go on for a few feet you will get clipped inside the rock and can't walk back out.

To exit you must continue on into the next rockface to the south and face the town, you should see the town behind a big oval shaped rock. There is a place where you can't walk, but if you crouch and crawl down the hill you will glitch through the wall once you've made it close to the edge of the rock.