[Brian328i]

From my understanding, isn't that was the ice walls are designed to accomplish? I understand that that would create a walled structure leaving the bottom open. I assume though that the shale and rock would maybe act as a liner on the bottom? I'm sure the engineers thinking of this already figured that out.

[SAJ]

Perhaps that's it. If SOME layer of shale will or can be forced to act as the ''bottom'' of the ''cauldron'', then the deal is sealed.

I think you're probably spot on or very close. I should have thought it through a bit more, evidently.