Once you got out from your base to fulfill the supplies of wood or limestone, you may run into ruins of houses with hatches. They lead to underground shelters with new kinds of enemies.
Chances to find the entrance are not too high, but as soon as you see the ladder sign on your radar - that's it!
Please note, that some hatches require special skills. Basements can be spotted anywhere, but remember: if you die there, you won’t be able to come back.