Basically how this works is the higher up you build the closer to the bottom you get. The problems you will run into building your map at a negative altitude (or depth) is that players can see what your saying at higher altitudes if they are in your location but at a positive location; if you've ever tried to make a basement directly under a house, then you know what I am talking about - players above can see what you type below and vice versa.
Conversely if you are talking about water... especially transparent water with sea floors, now you get a situation where you can "talk" type under water and players on passing ships can read what you are saying... lol.
I don't think this is client-side, but as Zavreyon mentioned, it could be. My ideology surrounding this stands to reason because the "globe" wrap-around for boats is done via the BaseBoat.cs; edit that file and you are done when modifying wrap-around points. So I think there might be a script that defines world elevation, but I do not know which one it is. Your other alternative, if it IS client-side is to use the ClassicUO client and modify it. That should extend your currently defined elevation points as you wish. The only clients this affects are the official ones.
In my own opinion, I believe using the official clients is a more logical way to go and just build things to scale using whatever depth you want as your facets defined surface point as GoldDraco13 pointed out, it will give you more height; just make basements and such in a different part of your map or on another facet entirely to simulate going into a private area. The official OLDER clients give an added advantage to this because you can use UltimaLive, by Praxiiz to add up to 249 additional worlds. What I have done with this is allow for each playable world to have 2 support worlds, one for mining and one for dungeons and basement type areas, and custom player housing interiors. I do not know if ClassicUO is compatible with UltimaLive, but I do know that UltimaLive stopped working after client 7.0.50.0 and Praxiiz never bothered to update it for unknown reasons.