During chapter 2 - Darkwater Port - in Call of Cthulhu, you will be given the task of getting into warehouse 36. This task is not easy - one entrance will be guarded by a policeman, the other by a pair of thugs who will be quite unwilling to let you in. There are two solutions to this problem - both described in this chapter.
Get inside warehouse 36 - getting past the thugs
One of the roads leading to the warehouse will need the help of two drunken sailors. You will need some liquor to convince them. To get it, go along the shore in Darkwater - avoid the people gathering around the dead whale and head to the shacks visible in the distance.
Enter the first shack by opening the door with lockpicks - inside, you will find a Bottle of liquor.
Now go back to the drunks and start a conversation. At the end of the conversation you will be able to give them the alcohol, in return they will draw the attention of the guards, thus opening a way for you to slip through. For your efforts you will receive an additional achievement - No prohibition in Darkwater.
Sneak through to the place guarded by the thugs. Go to the end of the path and then interact with the fence to open a hidden passage. You can now try your hand at opening the lock on the door - most likely your Investigation ability is not high enough and the plan will fail.
As the door can't be opened again, go back to where the Mysterious Woman, or rather Cat, as you probably already know, will be waiting for you. After a short exchange of courtesies, Cat will knock out the main character, who will wake up in the street.
When you regain control of your character, go back to the bar and talk to Cat. After a short conversation, the woman will agree to let you into warehouse 36 and then take you to the place.
Get inside warehouse 36 - underground tunnel
The second way to get into the warehouse requires you to solve a logic puzzle. Get out of Captain Fitzroy's office and go deeper into the city - a large group of people who had previously blocked access to that part of the city will be coming from the opposite side.
There you will find a small hall with a trapdoor blocking access to the lower level. You must find three items here. First collect the Lever - this one is located to the left of the entrance to the room, behind the crate.
The second item is the Ratchet Device - you'll find it near the lever, behind the broken cart.
The last item to be found is the Cogwheel - it can be found in front of the entrance to the building, nearby the ladder leading to the upper floor.
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 trapdoor in the floor.
Climb down the ladder and then enter the room with gas. Now you only need to go through the flooded cave - along the way, the hero will be dragged under water by a monster, and return to the surface after a short while. For this, you will receive The Delusional Detective achievement. After a short walk you will reach the interior of warehouse 36.