This is a big trench wars arena I made just before v9 came out, It has never been used and with the way things are going probably never will be, but if you have a copy of trench wars feel tree to use this with it. And if you like I would love if you send it to me.. :D
Anyway this is about as big as you can make without the server lagging, and I think there is an admin base deep at the bottom of the ground. As it was never used I cant really remember. It was built to work with trench wars and team death match, so I guess it may never work again.
The room:

How deep it is:

View from the sky:

Some diging:

Oh and you can download it
hereAnd put it in the slate save file folder.