By Bill Hayton
BBC News, Viengxay, Laos
One of the most secret sites from the Vietnam War era is being opened to the world.
The caves are where America's enemies plotted its defeat
The caves at Viengxay, in north-eastern Laos, once hosted the country's communist revolutionaries as they plotted the final US defeat in Indochina.
Even in peacetime it takes two days to drive to Viengxay from either Vientiane, the capital of Laos, or Luang Prabang, its second city.
During the secret war in Laos it was almost impossible.
The remoteness of this "hidden valley" was one reason that the communist Pathet Lao chose it as its headquarters. The other was that the valley is full of natural caves - nearly 500 of them.
Onechanh Somany spent nine years living in the caves during the 1960s and '70s as a communist policeman, enduring constant bombardment.
"Every day from 7am until 10pm the Americans bombed us," he says.
"At night they tried to blow up the bridges and the transport vehicles. There were brave soldiers on top of the mountain trying to shoot them down - many of them were killed. But inside the caves we were safe and no-one died."
War rooms and theatres
Per head of population, Laos remains the most heavily bombed country ever. In a nine-year-long undeclared war the US dropped half a tonne of bombs for every inhabitant.
It was a desperate attempt to prevent the North Vietnamese communist forces using Laos to supply their forces in South Vietnam and to prevent the Lao communists taking over the country.
Onechanh Somany spent nine years living in the caves
But it failed and in August 1975, a few months after the falls of South Vietnam and Cambodia, the US suffered its final defeat in Indochina - a defeat orchestrated from the caves of Viengxay.
At one point, there were around 23,000 people living in the caves. And the community had everything from factories to make uniforms, to hospitals, to rooms in which the war was actually planned.
Somkhid Bouaviengxay, one of the newly-trained tour guides, showed me around.
Not all the caves are open, but it is possible to visit the leaders' caves and some of the other more significant places - such as the theatre cave where visiting song and dance troupes from friendly socialist countries came to entertain the revolutionaries.
Conditions were basic at best, even for the senior leadership. The Communist Party bosses - the politburo - lived on camp beds in one room, while the leader of the party, Kaysone Phomvihane, had slightly more spacious accommodation through a tunnel which he shared with his children.
At the back is the emergency room.
"If the American aeroplanes had dropped gas outside," Somkhid explained, "everyone would have had to go inside [this room] and close the airtight door".
"Inside, they would have had to wind the Russian-made oxygen machine by hand to draw in and purify air from outside. But the Americans never dropped gas, so this room was never used."
Each of the caves has a story to tell, but all of them demonstrate how difficult life was during the war. One woman I met in Viengxay lost eight of her 10 children from fighting, disease and malnutrition.
After the war the area remained off-limits to foreigners because it was used to "re-educate" senior officers of the former anti-communist army.
The valley's caves and its remoteness made it an ideal base
Onechanh Somany told me that after the fighting he had worked in the re-education camps.
The legacy of the war years still makes life difficult today. Bounby Lasysamay, who works for the Dutch aid organisation SNV, which is helping to develop the caves into an attraction for foreign visitors, explained the problems.
"Laos is the poorest country in South-East Asia, and this is the poorest region of Laos," he says.
However SNV, and the local community, are hoping that by opening up the former "secret city" of Viengxay to travellers, the area will have a chance to benefit from what, until now, has been an unhappy legacy.
Things are already starting to happen - the valley is getting noisier with local people building small guest houses in the hope that visitor numbers will rise from their current maximum of around 20 a day.
And the country's tourism minister, Somphong Mongkhonvilay, himself a veteran of the caves when he was a revolutionary, will be happy to see as many visitors as possible - even the Americans that once tried to kill him.
"Vietnam has revealed many historical places to the outside world but in Laos we kept them secret for some time. But now it's time to reveal this secret city. That is why we would like to welcome all tourists from around the world - including those Americans involved in the war in Laos during that period."
As in Vietnam, the Communists of Laos spent decades fighting off foreigners and after independence came years of isolation and poverty.
But it is that lack of development which now attracts increasing numbers of visitors to Laos. The challenge is to turn this welcome invasion into lasting benefits for some of the poorest people in Asia.