The Meaning of Life, the Universe, and Everything.
This might be the largest underwater Minecraft world built in the console version of the game entirely in Survival mode and by a single person. It was built originally on the Xbox 360 version (starting in January 2014) and later imported to the Xbox One. On the Xbox 360 version of my world, there was not a single block of any kind above the water line (the seed was 98% water and I destroyed the remaining islands down the sea floor (they became part of my underwater buildings). When new players enter the world, they spawn on the ocean surface but immediately drop down into an underwater dome, so even the spawning effectively takes place underwater. This is not a world made up entirely of glass structures--the water is held back by glass blocks, glass panes, iron bars, wood fence posts, netherbrick fence posts, signs, ice, and ladders in different combinations as I experimented to determine all that is possible in underwater construction. The world contains some rather epic constructions, in my view (but of course I'm biased).