The orange beacon atop the conning tower came to life Sunday as the prime minister's wife Gursharan Kaur pressed the button to symbolise the launching of the vessel into water. On the conning tower stood the commander of INS Arihant Captain Anshuman Dutt.

In the middle of the body -- comprising an outer hull through which the water goes in and and an inner pressurised hull -- on the starboard side are two rectangular vents, meant to take in water when the vessel dives into the sea. It is like a 'cocoon within a cocoon', explained an official.

INS Arihant is longer than any of the submarine in the Indian Navy's fleet so far. A nuclear submarine is powered by a nuclear reactor, which generates tremendous heat driving a steam turbine. It has unlimited underwater endurance and speed twice that of its conventional counterparts.

'Miniaturising the nuclear reactor was a big task and if you want people inside the vessel, it has to be self sufficient in terms of energy and power,' V.K. Mehra, director, Reactor Project Group, Bhaba Atomic Research Centre, told IANS.

The submarine can carry 12 nuclear missiles K-15 and is the sea-leg of the nuclear troika of India -- capability to launch nuclear weapons from land, air and sea. Keeping in line with its 'no first use policy', the submarine will help India in developing a 'credible second strike capability' in case of nuclear attack, said officials.

Indian Navy has been operating conventional diesel-electric submarines, which have to surface to charge their batteries.