Anything static can be placed underground. On the whole map as Syd said, but you have to ask what's the reason why. Does having water over the "underground" area benefit anything? I would just treat it as a dungeon area, or if it'll fit, just use the normal dungeon area on the map.
The thing to remember is when you are in game, there is technically no "ceilings". The only use in making something under the land tile (map0) is in simple mining caves. Any other use is overkill. Use the dungeon part of the map. If you need more, then use other parts of the map ans a dungeon area. The only restriction is the X=5120 "line" that the client doesn't draw anything through.
As an example, look at the Darkshadows map in the screenshots page. Most of the map is actually just black dungeon areas, where you can do anything "underground" that you want.
Another limiting factor is when you make the "ground" or "dungeon floor" if you use statics, they do not stretch at all. This isn't good for "natural" floors. You're better off using the map0 tiles that do stretch to achieve a good looking effect. Map tiles of sand will flow to create decent looking hills, whereas sand floor statics will not connect correctly if you try to make a hill.
-Ryandor
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