Indian Navy personnel walk past a guard at the naval dockyard in Mumbai
The explosion on INS Sindhurakshak, one of India's frontline submarines, will cost the Indian navy heavily at a time it is looking to expand its naval defence capabilities.
The vessel exploded late last night, and it took over four hours to put out the fire that followed. 18 sailors are feared dead.
The navy, which is investigating the explosion, fears an onboard fire may have set off the highly-explosive weapons in the sub. Sources say the sub was being readied for an operation, which is why it was armed with weapons such as heavy-weight torpedoes and 'Klub' anti-shipping and land attack missiles.
The Sindhurakshak is widely seen as India's most advanced diesel-electric submarine, designed and built for anti-submarine and anti-ship warfare, defending naval bases, securing coasts and sea lanes of communication, reconnaissance and maritime patrol operations.
The Indian navy has been aiming towards more than 30 submarines to strengthen coastal defence, but it has only 14. The 16-year-old Russian-built Sindhurakshak was among these 14. No new sub has been inducted in 16 years.
The Sindhurakshak was handed over to India in January after nearly two years of overhaul and refitting in Russia at the cost of 480 crore rupees. It was expected to serve the navy for at least 10 more years.
In 2010, a sailor was killed when a fire broke out on the Sindhurakshak at Visakhapatnam port in Andhra Pradesh. The fire was blamed on an explosion in the submarine's battery compartment. Later that year, the vessel was sent to Russia's Zvezdochka shipyard for a complete overhaul.
It was refitted with improved electronic warfare systems, control systems, and integrated weapon control system.