What about towards the middle of the peninsula?

Also, when I was in New Orleans, they had a technique of driving steel plates/walls down before digging to dam back the watery dirt, and create a "tunnel" to excavate out the sludge. If the utility crews could regularly get down 20 feet with relative ease, I would assume the fortunes behind the Cabal would be able to get down as deep as needed to start boring into bedrock, making FL an even better target ... no one would ever think of looking there.

And how far down is the bed-rock? A couple hundred meters?

Sure, it's untenable for civilians. But if you have the Rothchild's money...