Go upstairs in the bunker, and look along the far side wall
for 3 levers on the floor. IIRC there are 2 levers near the desk where the jewellery merchant is, and one over towards the chest.
There is a lever handle in the cave under the waterfall, which you need to proceed there. Another can be found in the mine in Broken Valley (after getting the key from the guild master trapped in the quarry tower), which opens a room with some random loot. There is also a third lever handle, in a cave you don't have access to yet.
