Cistern - Filling the Cistern - Second Silver Key
Climb out of the pool at the base of the ramp, on the east side between the two gargoyles.
Climb onto the walkway above, then onto the onto the square pillar at the corner of the walkway. Jump back onto the balcony and go up the stairs into the ROOM WITH METAL GRATING AND CISTERN SWITCH.
Hop over either of the openings in the floor. Now pull the switch to flood the area.
Return outside and swim down to the north end of what was the original pool...
...through the small opening where you came out earlier.
Follow the passageway until you can surface. If you only killed one of the 2 rats (24-25) earlier, climb out on the ledge with the dead rat and shoot the other one from there. Pick up a small medi pack (16) (and a save crystal in NG+) on one ledge, a large medi pack (17) and shotgun shells (18) on the other.
NOTE: If you don't already have the SHOTGUN, you'll find it here in place of the shells.
Jump back in the water and pull the lever to open the door below. Swim down and pick up the SECOND SILVER KEY (19). The outer door opens automatically. Swim through and return to the surface.
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