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Not thinking that is possible yet.
The only solution I can think of is to in the setup.inI file set the water height to 50 and the terrain height to 100.
In a model program.
Make or have a terrain made of the ground layout you want using polycollision of 1.
Cut out the openings that you want to access your subterrain stuff.
Or you can jump to a different map when wanting to go underground and build your subways on the terrain.
When you want to come out of the sewer, then you switch to an above ground map.
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