The Panikotha Fort or Old Diu Prison or Fortim-do-Mar is a massive structure built on 56,736 square meter area. It was constructed between 1535 and 1541 AD to defend Diu and Gujarat from a possible Mughal invasion. The Sultan of Gujarat and the Portuguese rulers of Diu entered into alliance to jointly construct this fort. It is connected to land by an undersea tunnel which is now in ruins and protected by moats. It is about a mile from the Diu jetty and also has a light house and a small chapel dedicated to 'Our Lady of the Sea'. This tiny island, Panikotha, stands in the middle of the bay at the eastern end of the island. The lighthouse is modern, and there probably are pre-European fortifications on the island, but the visible fortress is Portuguese. Located in the middle of the sea, the fortress creates a beautiful view whether seen from the jetty, from the Fort, from the Village of Ghoghla or from Diu proper. Special light effects at night give it a mesmerizing beauty. Legend has it that this ship shape structure was one connected with land by an under sea tunnel. For sometime it was also the historic Diu prison in the middle of the sea.