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Poseidon Energy Reservoir not only distributed water to the surrounding farmlands and towns, but it was also a source of hydroelectric power. The power generated was used by Poseidon for its research facilities scattered about the Commonwealth. Currently, the reservoir is in a state of disrepair. The hydroelectric generators no longer operate and the water has become stagnant.
The roof gantries are easy to access, and the feral ghouls up here can be wiped out while you gaze south at Natick and Lake Cochituate. Inside, the terminal in the first metal office pod opens the security doors leading deeper into the facility. The Poseidon Reservoir Safe Key is on top of the metal pod, near a locked toolbox (Expert): at the top of the metal walkways, jump to the left and over the pipe. In the back left is the toolbox and the key.
- Robot model kit parts in a Railroad stash (indicated by the railsign) via a brown pipe going into the lake, accessible via an underwater breach in the pipe, outside next to the factory.
- Poseidon Reservoir safe key - Behind a large pipe in the upper west section corner (drop down from the railing). Opens a safe and a tool case.
- Cleansing the Commonwealth - This is one of the locations Knight Rhys will ask to cleanse of its inhabitants.
- Ghoul Problem
- Last Voyage of the U.S.S. Constitution - This is one of at least two destinations to which the Sole Survivor can be sent to retrieve the Poseidon radar transmitter.
- Learning Curve - This is one of the locations where you need to escort a scribe of the Brotherhood.
- Quartermastery - Scribe Haylen asks you to retrieve a haptic drive from this location.
- Randolph Safehouse - This is one of the possible locations that may need to be cleared out for Mr. Tims.
- Lost Soul - This is one of the possible locations that you may be asked to rescue a synth by the Railroad.
The Poseidon Reservoir only appears in Fallout 4.