50 to 85%??
Yup, that's "mud" to you and me. That sounds pretty perfect. It should be possible to tunnel through that, using only heat. and some kind of pump. What I propose (I could do with a BIT of funding, I'll set up a Kickstarter thing or something) is: Land on surface of mud-lake. Drop something hot onto frozen mud, tied to a string or something. (I'll flesh this out with details later) Pump slurry out of hole and away.
Lower the hot thing (I'm thinking a sub-critical lump of plutonium, heavily shielded) gradually. Keep pumping that mud. Keep going until you reach bottom, but pump a bit slower to create a chamber. Plug the shaft (using mud, now liquid, it will set like concrete!), but install doors in the plug in a couple of places. Now set up camp in your pressurized, radiation-shielded "warm" (-1 or -2 C) cave. Easy access to water. Tunnel horizontally outwards when you need more space. Refreezing bulkheads as you go.
I know I am sounding a bit flippant about this, but this could really be a game-changer if we are serious about colonizing Mars. This makes it possible. Up until now, a permanent self-sustaining colony has NOT been possible.
Biting the hand that feeds IT
