Recently, an old-school tabletop roleplaying site hosted a one page dungeon design challenge. Unfortunately, I didn't have time to craft an entry, and the deadline passed.
However, due to some confusion, the deadline was extended, and I was able to scrape together some time to make an entry of my own: The Poseidon Adventure. It's released under the Creative Commons Attribution-Noncommercial license.
The dungeon itself is a much abbreviated version of an adventure I ran quite a while ago (one of the first I ran using third edition). Some might accuse it of being a device to deliver the punch line of a terrible, terrible joke (it certainly got some mighty groans from my players when I said, as we were putting away the game table, "thanks for playing...the Poseidon adventure."). But really, I think the adventure stands on its own, too. The bad joke is gravy.
If there is interest, I might post what I have from the original adventure, but the primary macguffins from the original adventure are retained in the one-page-dungeon entry itself: the good magic gone bad, the impending rise of a new evil demigod, the drastic change to the dungeon environment halfway through the adventure, and the final race against time to escape the ruins. (And the terrible joke, natch.)
The changing environment in particular made a one-page presentation difficult, since I had to present the environment in both contexts, one in each orientation. As a result, I went more "new school" and kept things short and more plot-oriented than some of the other entries I've seen, but I think the thrill of such a dynamic environment is worth the space devoted to it. We'll see if the judges agree.
Let me know what you think in the comments.