Pull the Plug

Pull the Plug is a side quest which occurs at the Thicket Excavations in Fallout 4.

Detailed walkthrough
Sully Mathis wants to unblock some underwater pipes at the Thicket Excavations. The pipes can be located by finding the bubbles on the surface and swimming down (water is irradiated, 10 per second). Each pipe can be fixed by simply turning a red valve located on the pipes. There are two exits from inside the dam, there is a lift and a stair case which are on opposite sides of the dam. The lift has a button at the top and bottom, having it in the right position prior to navigating can be helpful. Underwater breathing boosts via the Aquaboy/Aquagirl perk or consuming a mirelurk cake are really not necessary here, as the valves are not very deep underwater. The locations are in a triangular pattern; it helps to scope them out from above before diving in as the bubbles are clearly visible. A hazmat suit and/or some Rad-X helps counter the high rads.

After fixing the pipes and speaking to Sully, the player character must start the pump which will spawn two mirelurks, one of which is or may be legendary. The player may leave the zone and come back later or simply sleep 24 hours in the trailer right behind the big pump to discover that the lake has been drained and occupied by raiders. At the very bottom of the dig site the player character will find Sully (now a much tougher boss, having a minimum level of 30) and several mirelurks in pens which can easily be killed if the player character doesn't open doors leading into their enclosures. In many cases, a single frag grenade should suffice. Sully seems to be breeding the mirelurks, for unknown reasons.