¹     Human beings have long wanted to travel through the sea as well as on top of it. Many people toyed with the idea of a sub (under) marine (sea) ship. Such a ship would be a big plus in case of war at sea. But how would you make it work? Human lungs must have air. How do you breathe underwater? Besides that, how would you get the craft to sink and then to come back up? How would it move through the water?
 
²     The first designs for such a craft appeared in the 1500s. Inventors tried to solve the problems of air supply and movement. A type of sub called Turtle was built in 1776. It was used in the Revolutionary War. It didn't quite work the way it was supposed to. Still, it may have caused the British to loosen their blockade of the U.S. coast. In the Civil War, a Union ship was sunk by the Rebel sub Hunley. By World War I, fleets of diesel powered subs were in use.
 
³     In World War II, subs played a big cat-and-mouse game under the sea. Each side had worked hard to design a more deadly submarine. German subs were called U-boats. These sly predators cruised the ocean, hunting for Allied ships. U-boats spent most of their time on the surface of the water. Topside, they could run on their diesel engines. They could also recharge the batteries that powered the boat underwater.
 
⁴     When a target was spotted, the U-boat submerged. A periscope allowed the pilot to see above the water. Hidden under the waves, the boat would slip silently up to its target. Then came the attack. A torpedo (underwater missile) was fired into the ship. The missile blasted through the hull, tearing the ship open. Fires broke out in many places. Sailors were dumped into the icy water.

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