I think it would not be necessary to terraform the entire planet, if the
Mariner Valley is indeed deep enough and able to hold in most of what's put in
it, it has enough area inside by istelf to be quite enough living space to hold
plenty of generations of colonists.
What are the physics of this? How much "leakage" would have to be overcome?
Would it have to be "roofed over" to hold in enough pressure?