I also have a follow-up question, according to the documentation
Destroy Basement Cell When Surface Foundation is Destroyed
true
If true, then when a surface foundation is destroyed, the corresponding basement cell (if it exists) will be destroyed as well.
If false, then when a surface foundation is destroyed, the corresponding basement cell will NOT be destroyed. The only way that basement cells get destroyed is if all entrances connected to that cell are destroyed. When set to false, this makes basements a lot more powerful, as you can do things like have a small base on the surface and a massive one underground or connect two distance bases via a basement tunnel.
But I do not see how you create a "Basement tunnel" because it only lets you drill out your entrances as far as there is a foundation correlating above it. Would you be able to clarify this?