The Great Siege Tunnels in Gibraltar were the starting point for the estimated 35 miles of military tunnels beneath the Rock.
The Great Siege oif Gibraltar by the Spanish and French lasted from 1779 to 1783 and the siege tunnels were excavated from 1782 to provide artillery positions against the attackers. Originally the plan was merely to provide access to an area known as the Notch but when ventilation portals were opened up during blasting it was realised the tunnels themselves would make an ideal platform.
At the end of the siege, the tunnel galleries stretched 113 metres and housed four guns. This was known as the Windsor Gallery, to be followed by the Kings' and Queens' Lines lower down the rock. Further chambers were excavated over the years and eventually the complex was incorporated into the rest of the labyrinth beneath Gibraltar.
The tunnels can be accessed as part of the Upper Rock Nature Reserve.