The submarine has been one of the most important inventions man has ever seen. A submarine is a vessel that can travel on the surface of the water as well as underwater. It is shaped like a cigar, and can be tightly closed from all sides. There are different types of submarines, which can travel underwater at different depths. They are usually powered by diesel-electric engines, but some are nuclear-powered also. Man has always wondered about the secrets hidden in the bottom of the sea. He has long made efforts to reach the bottom of the seas in search of diamonds and pearls. In order to succeed in his attempts, he tried to invent some device that could travel under water. French author Jules Verne wrote a novel called 20,000 Leagues under the Sea more than 150 years ago, in which he described what was obviously a nuclear submarine — even though nuclear technology and submarines themselves had not been invented then. Could this be a case of ESP? Nevertheless, Carnelius Van Drebbele of Holland devised the first submarine – a boat capable of travelling under water – in 1620. This submarine was made of wood and was wrapped in leather. It could submerge only to a depth of 3 to 4 metres in seawater. After that, many efforts were made to develop other types of submarines up to the end of 18th century. By 1727, fourteen different types of submarines had been made in England alone. In 1880, a submarine propelled by a steam engine was developed. Later on, submarines powered by gasoline and electricity also came into operation. Submarines were successfully used in the First World War (1914-1918). During the Second World War (1939-1945), submarines powered by diesel were also used in sea warfare. Now nuclear-powered submarines have been developed. The first nuclear submarine was launched in 1954 by USA. It was named ‘Nautilus’, after the submarine of the same name in Jules Verne’s fictional work, 20,000 Leagues under the Sea. Modern submarines are made from steel or even titanium sheets, with double hulls to withstand the immense pressures at depths beyond 250 feet. They are equipped with many instruments like periscopes, sonars and radars. The periscope enables crew to see what is happening above the surface of the water, even when the submarine is submerged in shallow water. Sonars help in locating the other submarines and torpedoes. Submarines have elaborate arrangements for filtering the air, and for maintaining the temperature inside the hull, for the crew’s comfort. Nuclear submarines are free from the problem of smoke and gas generated by fuels. Submarines are used for various purposes. They are very useful in oceanography. Missiles and torpedoes are launched from them to destroy enemy ships. Some nuclear submarines are even capable of launching Cruise missiles and ICBMs even when they are submerged at great depths. Modern submarines are equipped to track and destroy enemy submarines. Every submarine has emergency equipment that helps its crew to escape to safety in case of danger, such as serious damage suffered by the vessel. Submarines are now a vital part of every country’s defence system. However, only a few advanced countries know how to make them.