I had a dream of one possible underground future and I'd thought I'd share it with everyone.
It reminded me of the Toren's passage computer game where worlds were layered on top of each other.
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First here are the technicals.
Land parcels would have two options and would let you lower the ground level.
1. "Allow underground objects" would disable the "bob to surface" feature and would change the ground to a 10m thick slab as far as the physics engine is concerned.
2. "Disable ground" would only be able to be checked if there were prims on the parcel and would create a square hole in the ground.
The square would be on 4m boundary's.
3. You could build down to -780m, but the view and features would change
4. Z axis from "Ground level" - 10m to zero meters would be shared
underground space with water, ect...
5. No objects below 0m would be sent to the client unless you are both standing on that parcel and below 0m. This does mean that your entire underground build must be on one parcel.
6. Lighting would be set as if it was nighttime below -100m
The horizon would stay a daytime view for sky boxes, but objects would be dark unless lit.
7. Water line at -700m
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Now lets see what kind of uses this would give you.
1. You could make a basement in a house.
2. You could create tunnels threw the ground for trains, ect..
3. You could have your own private prim island without worrying about peeping toms.
4. Moving sky-boxes underground would remove clutter from the sky and once again allow flying over the mainland.
5. You can still create Underground landscapes since they are generally closer. (allowing phantom mega prims would make it easier)