The Titanic sank because it hit an iceberg, which tore a hole in the side of the ship and flooded six compartments with water. Thirty-eight thousand tons of water rushed to fill the ship, which resulted in the ship breaking up and sinking.
The exact reason and way the Titanic sank is still under debate. For a while, experts believed that the steel was weak or that the rivets holding the ship together were low-quality. Examination of these parts largely disproves these theories. The same steel and rivets used in the Titanic were used in its sister ships, the Olympic and the Britannic, without any negative results. Although the Britannic sank when it hit a mine, the Olympic ran for 25 years. In the course of service, it was repeatedly hit by ships, including hitting and cutting a German submarine in half, without sinking.
Another point of contention is the way the Titanic sank. Some experts believed that as the ship filled with water, the stern rose out of the water at a 45-degree angle before breaking off. Physics and eyewitness accounts seem to disprove this assumption. The stern likely only rose 11 degrees from the water and sank at a shallow angle. It is possible that the passengers did not even know that there was trouble until it was much too late.