Sino-Soviet relations
The project was started in 1966 when Sino-Soviet relations dramatically declined (see also the Sino-Soviet split). To enhance China's national defence and prevent possible Soviet invasion and nuclear attack, the project was approved (directly by then-Premier Zhou Enlai) and undertaken in secret. More than 60,000 engineering soldiers of the People's Liberation Army participated in the construction of the base. The underground base was designed to be able to tolerate thousands of tons of TNT explosives and 8-magnitude earthquakes.