This sidequest doesn’t have a set start point, as it could possibly begin at one of many points on the map. Hidden throughout the game’s world are five Power Cells (not to be confused with those on machines), which will allow you to unlock and obtain a very powerful piece of armor. All of these Power Cells can be found inside of the ruins left by the Old Ones and if you explore the Ruins in the first area as adult Aloy, that is where you’ll likely find your first Power Cell. The Power Cells are in the following locations:

• Inside of the Ruins in the first area of the game; behind some stalactites/stalagmites.
• Inside of All-Mother and can be found during The Womb of the Mountain main quest. It’s in one of the side rooms.
• At the top of Maker’s End. Once you get to the top and learn some plot from three Hologram Datapoints, there is a spot above the door that opens. Climb to the top here to retrieve the Power Cell.
• Found in the Grave-Hoard ruins. After solving the puzzles to open the ancient door, go through it to find a side room with the Power Cell on the ground.
• Found inside of GAIA Prime during The Mountain That Fell main quest. After the bit with Faro and the scientists, go outside and to the zipline, but don’t take it. Instead, look for a path that goes down, which will lead you to the Power Cell.

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Apart from one (left), the remaining Power Cells can be found while doing main story missions.

Once you have all five Power Cells, you can find The Bunker northwest of the Nora Hunting Grounds. There will be two puzzles for you to solve inside, one based on clock times (military times) and the second based on angles. The first solution is Up, Right, Down, Left and Up (from left to right), while the second solution is Right, Left, Up, Right and Left (again, left to right). This will unlock the Shield-Weaver Outfit and end the mission.