For more than 30 years, Laos' "hidden city", home of its top communist guerrilla fighters, remained exactly that: hidden.
Now Laos is opening the secret limestone caves to visitors, taking advantage of a tourism boom in one of the world's few remaining one-party Communist states.
Tour guides and backpackers have replaced the guerrilla fighters who lived in the caves underneath and around the limestone mountains of Vieng Xay province in the 1960s, where they planned attacks against the Americans.
After the war ended, Laos kept these caves off limits.
Communist party chief Kaysone Phomvihanh, who later became president, established a base in the caves in 1964 and moved the politburo and central committee office there.
The complex even included an airtight emergency shelter with an oxygen pump in case of American gas attacks.
"It's just amazing that people could really live here and have their meetings, make their plans and create a government in the cave," said Pamela Sweeney, an American tourist.
The biggest cave, called the "elephant" cave, was used as a theatre where musicals and dance performances by local and foreign performers as well as rallies and meetings were held.
With only a rudimentary road system and 80 percent of its population living off subsistence farming, landlocked Laos is one of the region's poorest countries.
But a boom in tourism is opening new sources of income and foreign exchange, with more than 1.6 million people visiting Laos in 2007.
Places like Luang Prabang, the ancient capital city of the Lan Xang Kingdom and a UNESCO heritage site, lure foreign tourists with ancient temples and panoramic views.
But even among locals, the caves, known as the "hidden city", remain something of a mystery.
"I'm taking my relatives to visit Vieng Xay district. We are very surprised to see the caves and to know the difficulties that our leaders experienced during the war," said Thao Pao Xiong, who lives near the caves.