Over the course of development, games go through numerous changes and overhauls. One modder has uncovered a lost Fallout 4 weapon that suggests some very intriguing things about the game's direction.
While everyone else picks over Bethesda's post-apocalyptic wasteland, modder xxdeathknight72xx has decided to look under it -- rifle through the couch cushions a little, see what they can find. Their first big excavation from the game's code? A harpoon gun complete with a bubble-spewing reload animation that suggests an emphasis on water -- perhaps even underwater combat. The released version of Fallout 4, of course, isn't really structured with that in mind.
Using xxdeathknight72xx's mod, you can try out the harpoon gun for yourself in-game. It's pretty low-res and has some animation issues, but it's definitely functional. It can even decapitate enemies and pin their heads to walls, where they will spin in a way equal parts gruesome and hilarious. Here's a video of Tyrannicon taking it for a, er, spin:
Perhaps the gun was scrapped as development progressed and Bethesda realised that basically nobody in all of human history likes underwater levels. Or maybe it's a scrap of something that will be patched or DLC-ed into the game at a later date. It's impossible to say at the moment.
Thanks for the tip, Geo Paradissis.