Hi, would it be possible to clear the underground first? So that there is no free space, but as an example at RimWorld you first have to dig through the area, clear it to be able to move freely there. If you place the stairs, a small area will be uncovered to be able to start.

A side idea here would be that you can find different resources from the rock you can cut away. There could also be different rock thicknesses, which in turn require different research.

One could also install ventilation shafts, otherwise the character could damage in the underground, suffocate. Ventilation shafts do not only bring air in, but also transport the pollution outside.