How2underground.

[General Nick]

Make a fake building with the underground in it, in the city, but a building that is non-enter able. That way you load it, people don't ask questions, and you have a "underground". Make it warehouse like.

Eh, if there's no other possible option then I'll probably resort to that.

The other purpose of the underground is a stream going through the campus.

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[Syntho]

Either do what Oasis said, or build the entrance to the underground teleport to the underground really far away. Jimmg did it, IceBlue did it, I do it, many others do it, why can't you?

[Chrono]

The best way is to modify the terrain and save it as a separate map that no one other than yourself. (or people who won't complain about crashes and fail to fix the problem and are helping you build)