A careless step could prove fatal when you inspect the ancient archaeological treasures of Laos.
You must be careful where you walk," says our guide, Lan Phetrasy, as he leads us around the rim of an enormous bomb crater and onto the mysterious Plain of Jars, the world's most dangerous archaeological site.
The track snakes off through the sunburnt grass up a hillside and is marked by hundreds of numbered wooden posts hammered into the ground. One side of each post is painted white, the other red. Step on the wrong (red) side and the last sound you hear may be the explosion of an American landmine.
Why would anyone risk their life to venture to the wild west of Laos's northern Xieng Khouang province? To see the most extraordinary relics of a vanished civilisation: fields of weathered stones that have been described as the Stonehenge or the pyramids of South-East Asia.
Scattered on the hilltops, under the shade of wild guava and crocodile trees, are thousands of enormous lichen-covered urns carved out of sandstone, some upright, some canted on their sides, split by encroaching tree roots. The largest are almost three metres tall and weigh about six tonnes.
Although it is 70 years since French archaeologist Madeleine Colani established a study institute here and first brought the jars to the attention of the West with her book Les Megaliths du Haut Laos (The Megaliths of Upper Laos), no one yet has any real idea who made them, or how, or what they were for, or what became of that ancient civilisation.
In part it is the remoteness of the site. No sealed roads connect the honky-tonk provincial capital of Phonsavan with the rest of Laos - you have to take your chances on a clapped-out Lao Airlines AR72 with broken seats and windows so scratched you can barely peer through them.
Flying in, you get your second clue as to why the world's most eminent historians are not queueing up to solve the mystery of the megaliths. The hills have been denuded of jungle cover and now resemble a golf-links pocked with thousands of dusty bunkers.
This is the dreadful legacy of the greatest aerial bombardment in the history of warfare. The Ho Chi Minh Trail, down which the Vietcong moved arms, men and supplies for their war with the Americans and their allies, ran through Xieng Khouang province and was subject to bombing on a scarcely conceivable scale for more than a decade.
That night, after a spicy stir-fry of hedgehog in its own bile and a glass of mushroom-flavoured lao-lao (the local rice wine) at the Phetrasy family's inn, we watch a video of a documentary, The Ravens - Secret War In Laos, which tells the story.
Between 1964 and 1973, the Americans dropped more explosives on Laos than they did during the whole of World War II - 2 million tonnes, which is the equivalent of one B52 mission every eight minutes for nearly a decade, or half a tonne of explosives for every Laotian.
Many of these were cluster bombs that opened in midair, releasing 670 bomblets, each containing a charge of explosives and 300 ballbearings. About a third of these failed to explode. Scattered over northern Laos are millions of these deadly "bombies", which continue to maim and kill at least one Laotian a week, many of them children.
Thanks to international mine-clearing teams, particularly the Manchester-based Mines Advisory Group, about 200,000 pieces of UXO (local slang for unexploded ordnance) are destroyed a year. But it will be decades before tracts of the countryside, including areas of the Plain of Jars, can be declared safe.
Lan Phetrasy, who is 29, and his father, Sousath, a war veteran who learnt his English from a downed American airman, have been largely responsible for opening up the Plain of Jars since the war, although only a trickle of 150 or so intrepid tourists make their way here a week.
Three "fields" of jars - actually rolling hillsides, pocked with bomb craters and slashed with the slit trenches and the foxholes in which the Pathet Lao soldiers hid while B52s thundered overhead - have been opened to the public.
But many more sites are waiting to be rediscovered. Colani located about 10,000 of these megaliths, but unfortunately she gave directions from villages obliterated by the war. Twenty entire villages have disappeared.
Only about 3000 jars have been relocated. The desperately poor country cannot afford the satellite survey needed to find the rest nor can it protect existing sites from vandalism and theft. The CIA even tried to airlift a jar back to its headquarters in Langley, Virginia, by helicopter as a souvenir, but gave up when the steel hawser began to saw through it.
Local legend has it a sixth-century Lao warrior, Khun Jeuam, had the jars made to brew rice wine to celebrate a war victory. But less romantic scientists believe they were the work of a civilisation that migrated over the millennia from northern India through South-East Asia to Indonesia. Similar urns have been found in Assam and in the Sulawesi Islands.
They appear to have been hollowed out by hand, using flint and bronze tools, which would date the urns to 2000BC, making them older than the Great Pyramid of Giza in Egypt and the Xian terracotta warriors in China.
What were they for? Artefacts - carnelian beads, bronze bracelets, ceramic pots - have been found nearby, but there are no signs of human remains in the pots. It is likely they were used for "secondary burial" rituals, in which the remains are removed after decomposition for cremation or burial elsewhere.
The sites remain sacred to the villagers. Until quite recent historical times - a few hundred years ago - Phetrasy says there were annual ceremonies where buffalo were slaughtered as an offering to whatever animist gods the locals worshipped.
And somewhere deep in the woods, at a secret location, Lan Phetrasy says he knows of a dozen urns still sealed with stone lids. They may hold the key to the mystery of the megaliths, but this will have to wait for a serious scholar with the time and money for a proper scientific investigation.
In the meantime, tourists are welcome to wander the Plain of Jars and let their imagination run free. If archaeology palls, there are plenty of other things to do. Many villagers grow silk worms and weave intricately patterned scarves, which sell for a few dollars. At Bau Noi, you can bathe in sulfurous hot springs. The local cuisine is cheap and spicy, like Thai food on steroids, and includes lots of "wild food" such as fruit bats, badger, termite eggs and swallows, which are netted when they fly in for dust baths in the bomb craters.
Villagers survive on subsistence agriculture and have ingeniously taken advantage of the tonnes of metal that fell from the skies: shops in Phonsavan sell ashtrays made of rocket-propelled grenade casings. At one Khmer village not far from the Plain of Jars, bomb casings support rice granaries, form fences and are used for pig troughs and planter boxes. The local blacksmith has made his forge out of war debris: the firebox is a Russian truck fuel tank, the bellows a flare casing, and the anvil where he beats out knives and hoe heads is the nose cone of a 450-kilogram bomb.
Nearby Tham Piu is probably the most sombre war memorial in all of Laos. Here, in 1968, a US fighter plane fired a rocket into a cave, suspected of being a Vietcong supply dump, where hundreds of villagers had taken shelter. A makeshift altar with a human skull on it marks the spot where 374 men, women and children were incinerated.
Phonsavan is not for everyone. The raw aftermath of war is everywhere. But the people are welcoming - Australians today are known more for the roads, bridges and schools they are building here than their participation in the war.
And the Plain of Jars is, quite simply, one of the world's great undiscovered archaeological treasures.
Getting there: Lao Airlines has two flights a day between the capital, Vientiane, and Xieng Khouang, the airport for the Plain of Jars. The return fare is $US96 (about $130).
Staying there: The Vansana is the best hotel in Phonsavan. It is modern, comfortable, air-conditioned and has a bar and restaurant. A double room is $US40 a night, including breakfast and airport transfers. See http://www.vansana.laopdr.com.
The Maly Hotel and restaurant is fun and funky and costs $US8 to $US30 for a room. See http://www.malyht.laotel.com.
Touring: A car, driver and guide to visit the Plain of Jars sites can be organised for about $US50 a day through a hotel or by contacting Lan Phetrasy at firstname.lastname@example.org.
Money: Only hotels take credit cards, and there are no reliable money-changing facilities in Phonsavan. Take cash - US dollars are best.
Safety: Be sensible about what you eat and drink. Use only bottled water. Have hepatitis A shots before you go to Laos and take anti-malaria pills. Use spray and mosquito nets. Pay attention to landmine warnings and do not stray off the beaten paths. The Department of Foreign Affairs and Trade says travellers should exercise caution. See www.smartraveller.gov.au.