MISSION 7: A CRACK IN THE SLABIn "Crack in the Slab", starting from the piano room where you get the time travel device. Make your way outside and turn right into the kitchen while you're in the past. Grab the key there and walk through the back door.
After jumping through the window there, stay in the present time and go through the way on the left. Walk downstairs into the basement left of the big hill of dirt.
Peak into the past and find 2 enemies looking at a hole in the wall. Take care of them in the past and grab the valve from the shelf next to them. Throw it through the hole and switch back to the present. Jump in the water, grab the valve and use it on the left side.
With the water drained, a box will lower with the hidden rune.