The Plain of Jars is one of the great unanswered questions of Asian archaeology. Who made the enormous megaliths, how they built them, and why, are mysteries which have long been the subject of speculation. Whatever their purpose, one thing is clear - they were the products of an advanced society which flourished in the mountains of northern Laos many centuries ago.

Yet the Plain of Jars is only the most famous of several sites which promise to yield rich treasures from Laos’s distant past. This much is clear from an archaeological exhibition which went on display in November at the Lao Revolutionary Museum in Vientiane. The organisers of the show hope that it will prompt further exploration of the prehistory of Laos, a subject which so far has been little researched.

The range of materials on display is remarkable. Glass beads tell of trade with India as far back as 3,000 years ago, while figures inscribed on 2,000-year-old burial urns suggest ancient links with China and Indonesia. There are stone age tools, iron age knives, and a bronze age axe head - in all, 200 objects from nine different sites, ranging from the banks of the Mekong in the country’s west, to the Vietnamese border in the east. The pieces date from as recently as the early centuries AD, to as far back as 8,000 BC.

"This exhibition will be the first of its kind ever organised in our country," says Thongsa Sayavongkhamdy, the Director of the Department of Museums and Archaeology. "The primary goal of this exhibition is to inform the public, to introduce the public to prehistory in Laos."

This is especially important because Laos may hold the key to many of the unanswered questions about the region’s early inhabitants, says Mr Thongsa, who unearthed most of the objects on display. "My wish is that archaeology should be developed in our country because the land is so rich, the land is so promising about new sites, about new discoveries, about many things that could eventually fill the missing links about the whole of Southeast Asian prehistory and protohistory."

Mr Thongsa has already made some progress towards solving the mystery of the Plain of Jars. "We presume that the Plain of Jars at Phonsavan was a necropolis, and it serviced a big city," he says.

Excavations at the Plain and at two smaller sites nearby yielded bones enclosed in ceramic urns, or in burial pits covered by tombstones. The jars may have been used as sarcophagi for the initial interment of important citizens, who were subsequently buried in the ground near the jars. Iron objects found during excavation date the civilisation from between the fourth century BC and the second century AD.

A distinctive figure inscribed on several of the funeral urns, known as the ‘frogman’, may link the civilisation to cultures as far afield as Yunnan and Indonesia.

A survey was undertaken to discover the site of the city which the cemetery serviced, and three ancient settlements were identified. In the course of the survey Mr Thongsa also discovered the quarry where the jars were carved, about six kilometres from the main site. It contained several unfinished jars which had been chiselled from stone using iron tools, which the society possessed in abundance.

Excavations in neighbouring Houaphan province, which borders Vietnam, showed the existence of another sophisticated society which existed around the same time as that which created the jars. Instead of jars, however, its cemeteries are distinguished by rows of upright stones - as many as 150 at the biggest site - and large burial chambers covered by stone disks up to two metres in diameter.

"That’s the only place such kind of tombs have ever been found. There are about 20 sites of this kind, all of them concentrated in the same province of Houaphan," says Mr Thongsa.

Excavations were also carried out at a cave called Tam Hua Pu near the northern city of Luang Prabang. While perhaps not as spectacular as the megalithic sites of Xieng Khouang and Houaphan, the cave is remarkable for the fact that it remained in use, either as a dwelling for hunter gatherers or in later times as a burial site, for at least 7,000 years, from 8,000 BC to at least the seventh century BC. Bronze and iron objects have been found there, as well as beads which indicate that the region had trading links with India.

The most recent site excavated was a necropolis at Lao Pako near Vientiane, dating from between the second century BC and the third century AD. The finds reveal that the inhabitants of this area possessed advanced iron making and weaving technology. Such finds and their relationship to each other raise perennial questions in piecing together the prehistory of Southeast Asia, says Mr Thongsa.

"Laos has a strategic position in Southeast Asia. Since time immemorial, Laos has been in an important position. It’s a crossroad, it’s central, it’s the heart. Not only for the position, but also because there is the Mekong river that can link such large areas.