Thanks for your interest! The bottom slab is isolated, and not connected to the side walls. It is just slightly smaller in size, so in theory the bottom could float up and down by any amount. You can see a dark line around the bottom edge which is two layers of heavy tar paper at the boundary between the edge of the poured slab, and the concrete blocks forming the side walls. In practice I don’t know if that made much difference in my situation, but it seemed best in theory. I also have several drainage holes at several depths, leading downhill and away at an angle, filled with sand. The top has gaps open to the side, but there is a crude tar-paper roof over the whole thing to prevent direct rain coming in. No water accumulation problem yet, even with our wet Oregon climate. It helps that the whole area is on a slight slope so there’s surface drainage as well. I wouldn’t have tried to put this down below a water table level; active pumping means too much maintenance in my opinion even if it was vibration-free.
One other comment: unless you can make it really tightly sealed, in my area I’ve found it better to make outdoor enclosures as ventilated and open as possible. If you can get some access to open air that seems to discourage ants who are always looking for sheltered dark dry spaces for their colonies. I had to wrap the mailbox post in sticky tape to keep them out of the mailbox.