While traversing Darkwater Port in the second chapter of Call of Cthulhu, you will find yourself in a room with a large metal trapdoor on the floor, which will block access to the underground tunnel. The tunnel itself leads to warehouse 36 and is an alternative route to this building. This chapter explains how to solve the trapdoor puzzle to get there.
When you are in a room with a flap, a total of 3 items are to be found. First collect the Lever - this one is located to the left of the entrance to the room, behind the box.
The second item is Ratchet Device - you'll find it next to the lever, behind a damaged wagon.
The last item that has to be found is a Cogwheel - it's located in the front of the entrance to the building, not far from the ladder that leads up.
When you get the items, climb up the ladder and reach the mechanism at the end of the platform. Interact with it and assemble the acquired parts in the following order: lever, cogwheel, ratchet device. Now activate the mechanism to open the floor passage, which will unlock after a while.
Now the mysterious passage will be opened. You can go downstairs and then enter a room full of gas. Now you just have to go through a flooded cave - along the way the hero will be dragged into the water by a monster, but after a short while he returns to the surface. For this, you will receive the achievement of The Delusional Detective. After a short walk you will reach the interior of warehouse 36.