F: Open the narrow door
You use 1 (ONE) LARGE KEY to open the narrow door, the key and lock disappear from existence just like the last one.
It opens to a room that doesn't have any mysterious liquid in it. Actually this makes more sense because there was a window where this door was supposed to be.
Maybe that hole in the wall is actually a portal?
F: Take harpoon gun and return to Victor with your info