I was always confused about how the TITANIC's watertight compartments made her "practically unsinkable." I always figured that even if just one compartment was opened up to the sea, it would continue to fill up, and nothing could stop it. Then the watertight doors would close, and the water couldn't spread that way. After this, the entire compartment would fill up, and it would spill both ways ( ------<->--------- ) into the other two. Therefore, I always thought that the ship couldn't have been unsinkable, and that the magazine Shipbuilder that was the first to call her unsinkable, had made a big mistake. However, there had been one thing a had left out of my theory. The pumps!

Imagine this;
a compartment somewhere in the middle of the ship is gashed open. Sea water pours in, but Captain Smith closes the watertight doors. The sea water is now knee deep, but the pumps are turned on. The sea water coming in, instead of rising higher and higher inside of the compartment, is flushed out of the ship through the pumps. The water inside the ship either becomes lower, or stays at the same level, depending on the size of the gash. (this isn't what really happened, I just made it up to explain to you how the compartments, watertight doors, blukheads, and pumps would have worked together to make her "unsinkable")

Also, if the foward four compartments are opened to the sea, she still wouldn't sink. This is because the bulkheads near the front of the ship were one deck higher than those near the stern, and in the middle.