The ship struck an iceberg

When the Titanic struck an iceberg, it tore into its hull. But the bulkhead compartments appeared to be holding the water back. 

The Titanic’s designer aboard the ship seemed to think the ship wouldn’t sink as long as the bulkheads held up - except that one of the main bulkheads had been damaged in the fire. After two hours, lead firefighter/stoker Fred Barrett reported seeing “green foam come tearing between the boilers.” Then, the bulkhead failed, and water came pouring in, sinking the ship.