The Titanic sank because it hit an iceberg and was not able to stay afloat as it took on water. The impact with the iceberg punctured the hull in five separate compartments that were not watertight; these compartments had space at the top where water could flow into the next compartment.
The Titanic had a total of 16 compartments in its hull, and four of these could be filled without the ship sinking. However, after the accident, more than four were filled, which caused the ship to begin sinking. It took over two hours for the ship to sink completely. During this time, the ship's bow sank, causing the opposite end to point up out of the water. This nearly vertical orientation resulted in the long ship breaking in half before both halves finally sank completely.
Since the ship was considered unsinkable, not enough lifeboats were carried for everyone on board. Because of this, only roughly 700 people survived the wreck, leaving over 1500 to die. No rescue came for nearly an hour and a half after the boat sank. When help arrived, a few people were saved from the water. The wreckage was not found until 1985, and after discovery it was examined for further information about the disaster.