This is very interesting news forward on WhatsApp by my friend Ananth. The tunnel 150 meters long, 3 meter wide and height of around 12 feet is found under Maharashtra Raj Bhavan.
The Bunker is spread over area of 5000sqft. Even more interesting was that "They didn’t find a single insect in the bunker"
Pictures are available here : http://www.hindustantimes.com/mumbai-news/not-just-governor-raj-bhavan-houses-a-5-000-sq-ft-british-era-bunker-too/story-BdUaTeJtbBBVzwqoQCe6jM.html
The bunker was perhaps built at the time of Prince of Wales's visit to Mumbai in 1875. There are rooms tagged Shell Store, Gun Shell, Cartridge Store, Shell Lift, Pump and Workshop on crumbling wooden plaques.