*These screenshots are part of the story anyway, so It is almost unmissable, I just added them to make it easier to know which part it's at*
I believe this is on the 4th/5th level, I'm not 100% sure as there is no chapter select.

Once you go through the Laboratory part of the level you will have to open a hatch and crawl down.


Once down the hatch, if you go to the first tunnel on your right there will be a part where you have to swim under water.
Continue swimming until you come to a door


Proceed to the back of this room and grab a spare valve.


Go back to the entrance of the room and fit the new valve next to the one thats already there.


When you turn the valve on the right it will unlock the gates that are directly opposite the door you came in. Quickly go out the door and swim straight forward and go into the open gate.

Once you come out the water this is where you will be.


go out of the door next to you and you will see an arrow pointing to go left.
Go right Instead


Now you will see this area.


Jump down off of that ledge and look to your right down the hall. You will see the skull.
Use Telekinesis to grab it.


Thanks for reading :)