Hey folks, I came up with an idea recently.
You know those fancy green lines on the civtransportmap that is located every 1000 blocks?

http://i.imgur.com/GtIaGRj.png

What if we all tunnel mined at those lines, but at Y12. It's not like it would be a chore or anything, as you'd still be able to find ores and veins and all that jazz. And if the idea catches on, we'd end up with a huge network underground, free of any obstacles on the surface.

Q: But I don't want any roads beneath my secret city!
A: Due to the huge quantity of these tunnels (if it catches on) and the fact that you don't see what's on the surface, it's probably even better for your secret hide-away that people walk underground than on the surface.

Q: What if I hit an ocean?
A: Either create a tunnel by placing cobble blocks, or make a staircase down to like Y3 or something where there is probably no water. Just remember to go up to Y12 again once the ocean is no longer an obstacle.

Q: So where do I mine again?
A: X or Z should have a clean thousand number. As in a number ending with "000". Basically 1000, 2000, 3000 etc... and including 0. But seriously just look at the /r/civtransportmap on those fancy green lines.

Now remember, the good thing about this isn't some kind of charity thing where you spend lots of time to better the server, which you could have used for other things. It simply takes a task that most people do, and instead suggests where to do it.