Originally posted by Ryzilynt:

Not too long after we discovered stone my buddies decided to dig out under the base to make a "basement". I was nervous that it would harm the structural integrity of our base above , but it didn't.

That's all-or-nothing. Tiny shards of the floaty rock will happily hover there and support your giant fortress 100% until someone hits the one cubic inch hotspot that was holding the entire boulder in place. Then bad things.