This underground structure, located beneath the Star Bastion, is the main gun powder magazine of the Galle Fort and is the largest. It is believed to have constructed by the Portuguese in the 16th century and was later upgraded by the Dutch to store gun powder and other ammunition for the cannons that were mounted at various points of the fortification. Subsequently , it was used by the British in the first half of the 20th century to store dynamite which was used to blast hazardous rocks along the sea access to the harbor. Light and ventilation for the  gun powder magazine was provided by two windows, one opening out to the sea to the south and the other opening out to the deep light well to the north. The two small openings on the vaulted roof of the main chamber allowed hot air generated within to escape.