I don't know. Myself and a friend infiltrated a server earlier this year and gained the trust of all of the regulars. At first we built normal houses and other random things. All the while, there were secret installations going in all over the map. Everything we built had a secret basement, concealed below the one open for all to see. Soon a massive underground complex was created with tunnels leading to just about every house on the server. Now, how could tunnels be offensive? An annoyance, yes, but offensive? They were secret NAZI tunnels. Remember how the levels in Wolfenstein 3D look? Yeah, like that but with more Swastikas and things I don't even want to talk about. We built these tunnels and waited for someone to find them. It was almost a week of solid building before another player hit one of the tunnels while building a basement. The reactions from the server regulars were hilarious. Their nice, clean server had been riddled with Nazi tunnels. They were so extensive that the admin tried to do a rollback on the sever, but we had been digging for so long that going to back up without the tunnels would have destroyed massive amounts of work. The best part? We just acted as shocked and offended as everyone else and no one ever suspected us.
EDIT: Oh wow, I posted this before bed last night and then was at work all day. I don't have any pics sadly, but I'll see if my buddy (also a Redditor) has any.
I also figured I'd give a few more details. The complex original started as an attempt to get as close as possible to the homes of other players. We figured we could dig in from below and make additions/deletions to their homes without them realizing how it happened. After a little while we didn't find that to be a great troll. We started expanding the tunnel network to an extent that we had to place markers at intersections in order to remember where we were heading. "At the three-way intersection take the path marked with a red flower to reach 'X' building and take the path marked with two yellow flowers to reach 'Y'" That sort of thing.
To gain entrance from our basements you had to know the exact block to destroy. Dig one spot to the right or left and you'd just hit dirt. The tunnels didn't widen until further in as to prevent accidental discovery. The entrances were typically marked with either bookshelves or a certain color block in a wall mural.
This theme continued within the tunnels themselves. To gain access to the deeper sections you had to know how to escape seemingly dead-end rooms. For example, my friend made a wall into a mural of an eagle. To continue into the tunnels you had to know to dig between the 2nd and 3rd claws of the left talon.
You see, this wasn't some kind of childish "lol nazis" troll. This was a slow-burn, "let's see how much we can get going just beyond their sight" sort of thing. It took on the Nazi-theme as we continued because we had to make sure when they saw it they would instantly know it was someone purposefully screwing with them. We didn't want them to respond, "Oh look, someone put a bunch of tunnels in our sever," we wanted the "Holy **** what has happened" kind of reaction.
You see, you never know what's going on just out of sight. What your neighbors are doing when no one is around. Sometimes even what your own family really thinks. So next time your suspicious of something, just think to yourself, "Maybe I'm not paranoid, maybe there are nazi tunnels."