I did more experimentation and zones also help you determine what is down there.
If the zone is entirely over open space or in solid ground, all the numbers will appear as "0". A tile is counted as hospital space, meeting zone space, pasture space, garbage dump space, or if it is ground. A tile is counted as a pit or pond if it is open space next to a floor tile. A tile is counted as Sand tile if it is a sand-bearing soil floor tile or wall tile next to a floor tile, and similarly as a Clay tile if it is a clay-bearing soil floor tile or a wall tile next to a floor tile. A tile is counted as a water source if it has drinkable water beneath it and a floor tile next to it. A tile is counted as fishing space if it has fish-containing water beneath it and a floor tile next to it.
It is possible to tell whether a tile is open space and whether it has water beneath it as long as it has a floor tile next to it.
Used in conjunction with the Road-dar, this will tell you roughly where the shores of underground water sources are, and where open space lies next to a floor tile.
An adamantine spire hollow section may give you 1-3 spaces of pit and pond and 1-3 spaces of meeting room, with a total number of squares between the two of not more than 4.
The zoning method may help to detect underground water sources.