Located approximately 70 kilometres north west of Hồ Chí Minh City but still within municipal boundaries, the Củ Chi Tunnel complex comprises over 200 kilometres of subterranean passageways constructed by the Việt Minh from 1948 onwards as an underground communication network during the Second Indochina War. From here guerrillas infiltrated Sài Gòn to carry out attacks or to spy on the American airbases that ringed the city. Two centres – Địa đạo Bến Đình and Địa đạo Bến Dược – are open to the public. In each case the principal tunnels are 60-70 centimetres wide and 80-90 centimetres high, with either a delta-shaped or vaulted ceiling. In places they were enlarged to form meeting rooms, kitchen areas and medical field centres. Above the tunnel was a layer of earth 3-4 metres thick, enough to sustain the weight of 50-ton tanks or heavy artillery and to withstand the destructive power of bombs of up to 100 kilograms. Specially-enlarged sections of the tunnels may be entered by visitors, who can enjoy a talk and video presentation in Vietnamese, English, French, Russian or Chinese prior to a tour of the site. The Bến Dược centre incorporates a war memorial and information centre.