Blue Prince: How to Drain the Reservoir Guide
Image Credits: Dogubomb, Blue Prince
The Reservoir is one of the biggest mysteries under Mt. Holly—and one of the biggest water problems too. If you’ve spotted the giant underground lake and thought, “That’s probably hiding something,” congratulations: you’re exactly right. But draining it takes a bit more than flipping a lever.
Here’s how to actually get it done.
Step 1: Get Access to the Pump Room
Before you can do anything about the water, you need the Pump Room. To even make this room appear, you’ll first need to draft a Pool. Once the Pool is placed, the Pump Room becomes a possible draft on that same day. Keep an eye out for it and slot it in when it shows up.
Read More: Blue Prince Tips and Tricks
Step 2: Activate the Reserve Tank
The Reservoir holds 14 units of water—more than the standard tanks in the Pump Room can handle. You’ll need the Reserve Tank, and that only works if the Boiler Room is active.
Here’s the catch: the Boiler Room has to be connected to the Pump Room using gear-type or red-type rooms. These rooms must form an unbroken chain, with visible ducts or ceiling shafts showing the path of power.
Once that connection is established, power will flow to the Pump Room and the Reserve Tank will light up.
Step 3: Start Draining
Now that the reserve tank is powered, it’s time to drain the Reservoir:
Select Reservoir on the control panel in the Pump Room.
Use Pump 2 to drain its water into Tank 2, and then into Tank 1.
Once Tank 1 is full, use the reserve tank switch to offload that water and free up Tank 1 again.
Repeat this process until all 14 water levels are drained.
If you need extra space, you can also divert water to rooms like the Fountain or Greenhouse.
Even if you run out of steps, your settings will save. The Reservoir will stay drained on future days as long as you don’t change the pump configuration.
Step 4: Explore the Reservoir Basin
Once it’s drained, head back down to the Reservoir through the Foundation elevator. Now you can walk across the basin and open several boxes and trunks scattered throughout.
Most require keys, so bring a few. Inside the crates, you’ll find cryptic clues—like “green notes are lies”—and rare items.
Step 5: Get the Sanctum Key
Your biggest reward is at the back of the basin: a black trunk with a flame symbol. Inside is a Sanctum Key, which is required to access the Sanctum later on.
You might also spot a white chest with a sun symbol that holds an Allowance Token—a handy item that lets you draft a free room.
Final Blurb
Draining the Reservoir might feel like solving a plumbing nightmare in the dark, but it’s worth the trouble. You get access to rare items, new areas, and an actual key to a major endgame location. Just don’t forget the Boiler Room—or you’ll be stuck watching water levels for nothing.
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