Mine for diamond at y=11.
On a quiet little server some of my friends from another community have been playing on for the last 6 months or so (on the same map), I've created a massive tower with floors each custom designed to fit a theme, under which I'm building an underground city through the TNT-widened mining halls I dug to get materials for the massive tower above. Everyone's stuff is connected by a fully automated rail system, so even when runecraft is down for updates, we can still zip around the server. Those things took a lot of community gold to make work though, after boosters were removed. I'll post some pix or vids later, but I'm certainly not giving out the server address here.