The vast Boston wasteland of Fallout 4 is littered with secrets and subtle easter eggs, which are always satisfying to discover. Even more satisfying perhaps, is finding something that the developers never intended players to find. One player had an idea while clearing out a raider camp, and decided to fall into the sea. Instead of heading to shore, he decided to explore the underwater depths to see what he could find. He eventually came across a mysterious underwater structure, that appeared to be some kind of water filtration system. The whole journey was documented on Imgur.
The player wasn't able to enter the submerged structure, but it did have a working light. This discovery, combined with a modder's recent discovery of a harpoon gun, suggests that Bethesda could be planning some underwater content for upcoming DLC. Either that, or Bethesda started working on an underwater Fallout 4 segment that was scrapped before release. Let's hope it's the former. I would love to see how the game mechanics of Fallout 4 would translate to an underwater area, especially if there is combat involved.