Find the Water Chip

Find the Water Chip is a main quest in Fallout.

Detailed walkthrough
Exit the cave and travel towards Vault 15. Enter Shady Sands (there is a green circle denoting a location uncovered during the travel to Vault 15).

Shady Sands
Locate a rope:
 * The easiest way to do this is to buy (or steal) it from Seth at the entrance to Shady Sands.
 * It can be found on certain bookshelves in Shady Sands.
 * It can also be purchased from certain traders. Many of these traders are located in the Hub.

After acquiring the rope, go to Vault 15.

Vault 15
The vault entrance is hidden in the uninhabited shack. Open the hatch and descend into the vault.

There are a few giant rats and mole rats in the area. Head towards the broken elevator and use the rope on the shaft to descend to the second level.

Scavenge the area for loot, including a Colt Rangemaster hunting rifle or H&K MP9 SMG, a leather jacket, and another rope (inside the locker). Use a rope on the second shaft and descend to the third level of the vault.

Proceed to where the command center should be. Once it is in sight, 500 XP is awarded for verifying that it is not salvageable.

The Hub
After coming out of Vault 15 empty handed, travel to the Hub. Ask Ian for directions.

Enter the large building labeled Water Merchants, and speak to the Master Water Merchant, Martha Rastello.
 * If pressed about Necropolis she will reveal its location. Alternatively, Miss Stapleton sells a holodisk with the location of Necropolis.
 * (optional) She can send water to Vault 13 for varying prices. This will add another 100 days to the time limit.

Travel to Necropolis.

Necropolis
Use the manhole to enter the sewers and proceed north through the sewers to the Watershed area. If the Vault Dweller takes this route, they will come across a group of friendly ghoul refugees and they can get information on the super mutants from their leader. After this, keep heading North through the sewers and climb up the ladder at the end. Find a building to the north with some super mutants and an imprisoned ghoul. In order to navigate the area without combat, either convince the super mutant leader Harry with a high Speech skill or sneak past. Violence is an option as well. If you fail Harry will either attack or send you to Mariposa Military Base.

After dealing with the super mutants, free the ghoul prisoner. If he is still alive, he will identify the entrance to Vault 12 in the adjacent cell. However, the entrance can still be opened without speaking to him.

Ghouls inhabit the Vault area and are hostile during nighttime. Travel to the third level using the elevator, and enter the command center area. Remove the water chip from the computer. It should be noted that doing this will cause the ghouls in the Vault to turn hostile.

After obtaining the chip, head back to Vault 13 and give the chip to Overseer Jacoren.

Rewards

 * 500 XP for finding the destroyed command center.
 * 7000 XP for giving the water chip to Overseer Jacoren.

Related quests

 * Destroy the source of the Mutants
 * Destroy the Mutant leader
 * Find the Water Thief

Bugs

 * To keep the water chip, simply press the 0 button when the Overseer asks the Vault Dweller for it. Afterwards the Vault Dweller will be at the computer, gain the 7500 XP, lose the time limit, get the new quests from the Overseer and keep the chip. If you have a bag or back pack below the water chip in the Vault Dweller's inventory list this will also keep the chip from disappearing.
 * There are two ways to "reset" the quest. The easiest is to talk to the Water Merchants. The other requires that the Vault Dweller kept the chip: returning to Necropolis and used it on the water computer, there by also undoing the chip quest, which means the Vault Dweller can give the water chip to the Overseer again for 7500 more XP. Using the cheat allows the Vault Dweller to keep the chip and then travel back and forth between the Hub and Vault 13, gaining 8500 XP each round trip and increasing the time limit all the while.

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