Lightningline,
Sometimes a picture is worth 1000 words, but I think not in this case. I hate to be so explicit, but here goes.
Complete spoiler.
spoiler:You need to go down from the water to the earth system. Usually, the first time, you find Earth by going down a ladder from one of the four water piers, from where they have a common platform, but it could be down an elevator if it is past 12:00 Factory time. The earth system has three or four levels, depending on how you count. You first arrive at the top level, where you find the arrow machine. From there, find a metal staircase in the center of that area. Go down a little way from the top to the next level down from the top - the metal stairs have an exit there. Go into a side room, bear left, turn right and see door 2200 across a catwalk.