Now, over 30 years since the conflict ended, the communist country has opened up the remote wartime hideaway to tourism, hoping to bring development to this explosives-littered and dirt-poor part of the country.
The network of almost 500 caves was home to 23,000 people and boasted all the facilities of a city, including not just bomb shelters but also shops, schools, a printing press and a hospital cave staffed by Cuban doctors.
Hundreds of troops and villagers could shelter in the cathedral-sized Elephant Cave, where propaganda movies were screened and visiting theatre troops from socialist countries performed to bolster battlefield morale.
Smaller caves connected by tunnels were the homes of Communist Party Chief Kaysone Phomvihane and his politburo, set near an emergency shelter with a Russian oxygen generating machine in case of a gas attack, which never came.
"This is the birthplace of modern Laos," said Siphanh Vandouayang, who spent much of his childhood here and now runs the local visitors' centre.
"Most of the members of the revolutionary leadership lived and studied here."
The picturesque landscape of limestone peaks and rice fields between 1964 and 1973 became the most heavily bombed place on earth as US jets targeted Pathet Lao fighters and North Vietnamese supply lines, he recalled.
