WTF is this thing?

Found this near the outskirts of Sandarian Caves. Crazy 1.8 artifact or man-made?

I'm pretty sure Sandarian and Dash were building Thunderdome last night.

HAHA YES. BREAK A DEAL, SPIN THE WHEEL.

That is indeed the Thundadome.

Rules of the Thundadome:
1. 1 Man-woman enters, 1 Man-woman leaves.
2. No food
3. No armor
4. Weapons decided before entering, and only the same weapons may be used. (Swords and Bows)
5. Participants may chose to store their items in the Participant's Shack near the entrance.
6. The prize will be decided upon before entering the Thundadome.
7. Once you enter the Thundadome, the other contestant must be defeated before you may leave.

The iron door is controlled by a pressure plate on the outside to let you in, and a chest of Redstone Torches have been left inside the door to act as a trigger to let you out. If there proves to be enough interest in keeping this up, I will go to some lengths to find reagents to make some higher quality items that I will personally put in the prize-box randomly.