Whenever I enter a basement it takes me to a randomly generated cavern one random map tile away from the intended destination. I end up having to jackhammer my way to the proper floor.
Same thing seems to happen in Strange Temples too.
Edit: It seems that whenever you enter a single-floor underground area using stair; the area around it is turned into randomly generated caverns with items and enemies in it. Have yet to test with labs, LMOE shelters, and bunkers.
Edit2: LMOE shelters seem to work fine; the caverns are still generated but you still properly appear on the stairs. The caverns seem to be generated endlessly. They all have a stairs up which leads to the surface but no way to come back down.