I don't have much more to say to this than I put in the title. I wonder why this can't be constructed. ( Wron, of course, I don't wonder, I think simply nobody as of yet had thought about this, though I think this is not that far.) There is this so called 'root cellar' in Sanctuary Hills. If you look out on Youtube for Sanctuary Hills constructions, this cellar is mostly outside of the defended area. With threats from around and sometimes from the air, people would tend to seek shelter underground. In the construction menu there is this floor tile with a stair upwards. It can't be used to lead downwards. I would like a similar item, that leads downwards; with surrounding walls, of course, that should be possible to push farther. I am not determined, that such basement should be part of the upper world, or something like the root cellar in Sanctuary Hills. This could be choosable. I just think in Sanctuary Hills, each house foundation should be able to provide a stair down to a cellar, and they could be connected, and they could have an own worldspace.