This is a sewer near Malden Center and north of Taffington boathouse consisting of a drainage area with a small overlook. The western wall has a small mag-locked room with an advanced-difficulty security terminal. The north end of the drainage area has a grate-covered pipe with a few radioactive waste barrels on the other side (up to ~27 rads).
The overlook can be reached by walking along the white tubing starting from the northwest corner of the room. Entering the area reveals the corpse of Russell Sutton, a chemistry station, shelves with various chems, a trunk with leveled loot, and a fusion core in its charging station.
If you leave the area by using the tcl command you will notice that the part of the map where the entrance is supposed to be is different from the same part that actually exists in the Commonwealth. There you can see how the map looked like in earlier stages.
Malden drainage appears only in Fallout 4.
PC Xbox One Tied to the "Guns doing no damage" bug. Entering Malden drainage and then exiting without opening the Advanced terminal may generate a bug that causes all guns to eventually stop doing damage. This includes NPC guns and lasers. Does not seem to affect Melee damage. This will affect all previous saves as well, so simply deleting saves with this event is not enough to clear the bug. Fix - delete all saves with this event, exit from Fallout 4. Physically disconnect Xbox from AC. Wait 30 seconds, and reconnect. Boot up Xbox. Load Fallout 4 and start from latest save. Bug should be gone. [verified]