The Key Location and the Door It Opens
In Old Mine, find the Mining Shaft Key on a body on a ledge partway along the elevator shaft that connects the minecart area back to the Lantern. It opens the door beside the switch used to redirect a minecart into a wall.
The route is in the Caverns during Chapter 4: Dark Descent, with Caverns > Old Mine as your Lantern reference. Beyond the locked door, another elevator leads down to a shaft containing a Ferrofluid Flask Power Upgrade in a chest behind a Volunteer.
Getting to the Caverns
If you have not entered the Caverns yet, speak with James at the Abandoned Church after rescuing Louis. He gives you the Cave Grotto Key, which opens the door in the cliffs across the grapple point. That gets you onto the route into the Caverns.
During the Caverns story route, you obtain Stasis at Weeping Ledge after using the crank to turn the rotating walkway. Old Mine also contains moving-platform puzzles, including a spinning platform you can stop and a lever-operated box you can freeze.
Find the Key on the Elevator Ledge
- Progress from the Caverns > Old Mine Lantern through the mine until you reach the tunnels with moving minecarts.
- Use the minecart-redirection switch to send a cart into the wall, opening the route toward the elevator.
- Continue through the opening to the elevator at the end of the minecart area; this is the shortcut lift leading back to the Lantern.
- Ride the elevator while watching for the intermediate ledge with a body, which is your exit before finishing the ride.
- Drop from the elevator onto the ledge with the body to reach the key pickup.
- Collect the Mining Shaft Key from the body so you can open the door beside the minecart switch.
Unlock the Shaft and Collect the Upgrade
Use the minecart switch as your landmark when finding the locked door again. You want the door beside the switch that sent the cart into the wall—the same wall opening that gave you access to the shortcut elevator.
- Return to the minecart-redirection switch to locate the locked door beside it.
- Open the nearby door with the Mining Shaft Key to access the elevator beyond it.
- Ride the elevator through the unlocked doorway to reach the lower shaft area.
- Follow the platforms down the shaft to reach the bottom, where a Volunteer stands near the reward chest.
- Open the chest behind the Volunteer to collect the Ferrofluid Flask Power Upgrade.
Using the Reward and Returning Later
After collecting the upgrade, apply it at a Lantern. Lanterns are also where you spend Catalysts on levels and equip Ambers, so use the stop to prepare before continuing through the Caverns.
You can return to the Caverns through fast travel for later exploration. For this key, select Old Mine and use the moving minecart tunnels, the wall-breaking switch and the shortcut elevator as your route landmarks.