Silent Stalkers of the Sea

Because they are meant to spend most of their time underwater, submarines are designed and built quite differently from other ships.

Submarines must be airtight so that water can't get inside them when they submerge. They also need to have strong bulls because the pressure of seawater at great depths is strong enough to crush ships. And submarines need special engines that don't use air when they are underwater. Otherwise, they would quickly run out of air and shut down! So most modern subs are powered by electric batteries when they're submerged. Some are powered by nuclear energy.

Because a submarine is completely closed up, it must have special instruments to act as its eyes and ears underwater. A periscope is a viewing device that can be raised up out of the water to allow the submarine officers to see what is around them. Another special system, sonar, 'hears' what is under the water by sending out sound waves that bounce off everything in their path. These echoes send a sound-picture back to the sub.

But why build submarines in the first place? Well, submarines hve been very useful in times of war. They can hide underwater and enemy ships by surprise.

Submarines have peaceful uses too. Scientists use smaller submarines, called 'submersibles', to explore the buge ocean floors and the creatures that live there. People also use submersibles to search for sunken ships and lost treasure. The luxury liner Titanic was discovered and explored with a submersible 73 years after it sank in the Atlantic Ocean.

The Nautilus, the first nuclear sub, was once caught by a fishing net. The fishing boat and its unhappy crew were towed for several kilometres before the situation was sorted out.