A submarine, often referred to as a "sub," is a watercraft capable of underwater operation. It is designed to navigate beneath the surface of the water, allowing it to perform various missions, including military operations, research, and exploration. Submarines can be powered by nuclear or diesel engines and are equipped with advanced technology for navigation and communication. Submarines are typically classified into two main types: nuclear submarines, which can operate for long periods without surfacing, and diesel-electric submarines, which require periodic surfacing to recharge their batteries. They are used by many navies around the world and play a crucial role in defense and intelligence gathering.