【多选题】Which are the fundamental characteristics of underground space?
A.
The underground medium is a precious space that can provide the setting for activities or infrastructures that are difficult, impossible, environmentally undesirable or less profitable to install above ground.
B.
The natural protection it offers to whatever is placed underground.
C.
The containment created by underground structures has the advantage of protecting the surface environment from the risks or disturbances inherent in certain types of activities.
D.
The opacity which t hanks to the natural visual barrier created by the geological medium.