Switzerland. Canton Lucerne. Medical exercice in the emergency hospital of the Sonnenberg tunnel in Lucerne during the largest civil defense exercise ever held in the country. A woman is lying on a bed with her left leg wounded. She need a surgery and is assisted by several nurses. From 16 to 21 November 1987, almost 1200 men and women converted a motorway tunnel into perhaps the world's largest bunker structure. The civil protectors had to prove during the exercise «Ameise » ( Ants in english) that in an emergency more than 20,000 inhabitants of the city of Lucerne could survive here in the mountain for two weeks. The Sonnenberg Tunnel is a 1,550 m long motorway tunnel, constructed between 1971 and 1976. At its completion it was also the world's largest civilian nuclear fallout shelter, designed to protect 20,000 civilians in the eventuality of war or disaster. Based on a federal law from 1963, Switzerland aims to provide nuclear fallout shelters for the entire population of the country. The construction of a new tunnel near an urban centre was seen as an opportunity to provide shelter space for a large number of people at the same time. The giant bunker was built between 1970 and 1976 at a cost of 40 million Swiss francs. The shelter consisted of the two motorway tunnels (one per direction of travel), each capable of holding 10,000 people in 64 person subdivisions. A seven story cavern between the tunnels contained shelter infrastructure including a command post, an emergency hospital, a radio studio, a telephone centre, prison cells and ventilation machines. The shelter was designed to withstand the blast from a 1 megaton nuclear explosion 1 kilometer away. The blast doors at the tunnel portals are 1.5 meters thick and weigh 350 tons. The logistical problems of maintaining a population of 20,000 in close confines were not thoroughly explored, and testing the installation was difficult because it required closing the motorway and rerouting the usual traffic.