Occasionally when entering the subsurface tunnel, the character will immediately die. Additionally Coral is generated in the subsurface layer, which looks odd. In my opinion a simple fix would be to enable surface pressure requirements for the drill, similarly to how AAI Miners restricts building on Maraxsis, or Vanilla Roboports.

Another odd bug is Maraxsis Big cliff explosives completely breaks the subsurface functionality.

What do the explosives do?
I cant test because I dont own the DLC. Please provive a screenshot about how a subsurface looks like

The cliff explosives completely remove all subsurface walls. The item is Big cliff explosives (maraxsis-big-cliff-explosives). In an Area of Effect size of 450 it destroys cliffs. However in the case of subsurface, all cliff walls are deleted, but the floor is not expanded. This leaves a subsurface that cannot expand anymore as there aren't any walls to dig out. I also can't throw cliff explosives at the surface edges anymore.

Ok will fix it