So a couple months ago, back around October / November, I ran a build server for the Penny-Arcade.com forums that hosted mostly megaprojects. One of the things we put together was the City of Rapture from the Bioshock games - an anachronistic underwater metropolis. We filmed a little machanima at the time, which you can view here: Part 1 & Part 2. I also did a quick flyaround recently since I got Roundaround's Crystal Clear Water mod working.
I dug this old map out of a hard drive I was backing up on the machine that used to host the server, and I thought I might post it. It was made back in Alpha, which is why you won't see some of the newer blocks, and also why we didn't do a whole lot with redstone - it didn't work at the time. It was done entirely by hand, including the mining and creation of materials, with the exception of recently using MCEdit to clear a spot on the ocean floor out to drop it into a new map by itself. We had Minecart Mania running on the server, which is how our whole metro/tram system operated back then. If you have an autocart-esque mod running, I don't see why it shouldn't still work - might just take a little renovating. It should spawn you by the Lighthouse - head on inside, and you will find the Boathysphere(tm) Ride, which was how we got into the city before minecarts worked in SMP.
I will probably be releasing some other stuff in the next week or two as I find time, including a flying city map. Enjoy!