The Chinese Nationalist government had moved their capital to Chungking [now Chongqing] in 1937, following the Japanese invasion. It was to become one on the most heavily bombed cities on earth, casualties were high since it was largely undefended. To provide air raid shelters a huge network of tunnels were carved into the sandstone cliffs upon which the city was built.
One of the most destructive raids was on 5th June 1941 when Japanese planes launched successive sorties against the city for more than three hours. When some of the tunnels became blocked during the bombing they became a death trap, asphyxiating as many as 4,000 people in one incident.
Contemporary U.S. newsreel of the raid: