The deeper you go, the more problematic the cooling and oxygen / CO2 recycling becomes. Not saying it can't be done. With a black budget, anything is possible.
I'd guess you wouldn't want to go more than a couple of miles down - which for most purposes, is undetectable. But what do I know?
Sullivan can't stonewall forever. He can only sit on it for so long. After that, it'll either be case dropped or extra fuckery of some kind. I've been thinking he'll try and send Flynn to prison.
It'll be an obvious miscarriage of justice, but if it gums up the wheels until after the election, it's job done for the DS.
Whatever leverage Obummer et al have on Sullivan, it will be applied to the max. He's stuck between a rock and a red scarf. Or a three-shot-to-the-back-of-the-head suicide.
I think the objective at this point is to prevent Flynn joining the Trump administration prior to the election, then hope to nick it so everything that's taken place in the last four years can be shut down.
There are 101 ways to piss the Russians off. This is one of them.