What I am about to tell you, is unspeakable. I have no idea how it happened, or why it happened. We have clear evidence that the incidents that occur in this story are legit, but we still have no idea why these things happened. Herobrine? Nah. This is something more serious. Nothing to joke about.
It all started one evening, when my friend (I will refer to him as "Jinjo") was exploring in my server. My server is a small, simple server, where a group of people play. No one ever moves too far from spawn though, they kept close to it, and made their bases there. My friend, Jinjo, decided to go exploring, saying he was going to build something big, and didn't want to be griefed. He moved away from spawn, and began walking in a straight line to the west.
I however, was not on the server. I was watching on the live "Dynamic Map" which is a common plugin for servers. It allows users to see what is going on in the server, without actually being in it. Buildings, players, terrain, and caves can be seen from the map.
Anyways, Jinjo began to see strange item drops laying on the ground. He claims to have seen a diamond just sitting there, and minutes after picking it up, it disappeared from his inventory completely.
"Strange." He said, but kept on heading away from spawn, in that general direction. I watched on the map, as it began to render new terrain. No player had ever been that far from spawn.
Here is proof that he was rendering terrain:
http://i.imgur.com/5UExn.png

He continued to find item drops, but no players to be seen. He found wheat, and some other items, just laying there. There was really no possible explanation, but neither of us thought of it much.
Finally, after following these strange drops, Jinjo had rendered an entirely new section of the map, which was far from ever being explored.
"I found a creepy shack," said Jinjo.
Indeed, he had. I looked on the map, and saw the strange building which stood tall in front of him. When he first found the building, there were NO torches inside of it. Here it is when he first found it:
http://i.imgur.com/VKZ4Q.png

Also, here is a screen of Jinjo, and the house, from the map:
http://i.imgur.com/SZDu4.png

There is no way Jinjo could have built this house in the time available. I was watching him the entire time on the map, and he never stayed in one place. I would have noticed. Also, there is no way any other player could have built this, for this section of the map had JUST been rendered.
Here is the inside of the house:
http://i.imgur.com/FE5X5.jpg

What is very strange about this, is that there were originally NO torches inside. Jinjo had placed the torched in the picture he had taken.
Another screenshot of the ceiling from inside:
http://i.imgur.com/T1WFh.png

"The two planks in the middle of the floor seemed out of place, so I removed them, and this is what I found." Said Jinjo.
Here is what he found in the floor, nearly after discovery of the building. There is no way Jinjo could have dug that deep of a hole in that amount of time.
http://i.imgur.com/EsDE1.png

Jinjo proceeded down the hole, with godmode, in fear of dying from the height.
Here is what the tunnel looked like, from the inside;
http://i.imgur.com/YKqY1.jpg

Also, here is a screen of what the cave system looked like from the house.
Notice, the cave system coming DIRECTLY from the house:
http://i.imgur.com/YHsGR.png

As Jinjo proceed down the cave, he found a MASSIVE cave system! It appeared as an underground ravine:
http://i.imgur.com/EJTJf.png

Here is a screen of the cave system from the map. I pointed out to Jinjo, that there was a certain area nearby that could not have been generated:
http://i.imgur.com/NSlJM.png

As he proceeded into that section, he started seeing something unusual. A certain material which could not have been normally generated in the over world.
Here are the two pictures of the entrance of this room.
http://i.imgur.com/5aNA6.png
http://i.imgur.com/Cqusp.png

And finally, the interior of the "rectangular room"
http://i.imgur.com/EHwbF.png

As you can see, the material in this room is glowstone. Contents of the nether. I don't get it at all. And that's when the server crashed.
"I'm done." Said Jinjo.

So was I, but I couldn't help scanning over a bit more of the cave system.
This is what I found:
http://i.imgur.com/GVuIP.png

I'm done with this now. Hit me up with a private message if you would like to visit my server. I'm not going to post it here.