The Titanic Mod brings people at one of the most detailed and accurate free 3D recreations of the Titanic ever made. It puts the player in a whole new storyline for the game Mafia.
The fore- and lowermost location on the ship, located in front of Cargo hold 1. In reality, this area probably had the least of care to it since there was no need for anyone to enter it besides maintenance. It stands in perfect contrast to the luxurious 1st class accomodations a few decks above. It's unknown to me what they used to fill this tank with. It could be water. If it was up to me I would fill it with Guinness.
This area might also look familliar to some Titanic fanatics who watched Channel four's "Titanic: The Mission", where they created a lifesized replica section of this area. I used pictures from it to use as reference. (Photo copyright belongs to it's respected owner)
They probably used it to balance the ship.
Peak tanks (be it stern or aft ones) can be used for fresh water storage or as ballast water tanks for balance.