I logged on this afternoon to find this:
(Ignore the floating blocks in the distance, they're just a graphical glitch.)
That's Sky City on fire. Here's another picture close up.
Somehow, a band of griefers called the Red Faction Miners found a way to use fire on the server and they burned down practically all of Sky City. I spent about an hour with a few other guys trying to put out all the flames, which was difficult due to all the floating blocks. It was like the aftermath of a Mongol horde, practically the whole city was destroyed.
My own home was fairly badly damaged, but I managed to repair most of it. They destroyed my chests in the house but, luckily, they were too stupid to find my hidden mine under a hill near my house, so all my gold, iron and diamonds were safe. All I need to do is fix one wall and get my flashing redstone lights working again, then my house will be good as new. But Sky City is absolutely ruined, and the mayor there gave up because of how upset he was.