Only one of the hundreds of stone jars at Site 1 of the Plain of Jars is decorated. It appears to show a frog-like creature that has been likened to a figure in ancient rock carvings in southern China.

New research indicates that the enigmatic Plain of Jars in northern Laos, adorned with colossal stone jars carved from sandstone millennia ago, served as a burial ground for an even lengthier period than previously believed—potentially spanning up to 2,000 years.

These monumental jars, likely employed to decompose the deceased until only their skeletal remains remained for burial, may exceed 3,000 years in age, as per recent analyses.

However, the findings also propose that the majority of human remains laid to rest adjacent to these ancient vessels date back to a narrower timeframe, ranging from 700 to 1,200 years ago.

“What we surmise from that is the enduring ritual significance of these sites,” University of Melbourne archaeologist Louise Shewan told Live Science. “They were important for a very long time.”

Since 2016, Shewan and her partner, Australian National University archaeologist Dougald O’Reilly, have dedicated their efforts to investigating the ancient stone jar sites. They collaborated closely with archaeologist Thonglith Luangkoth from the Laos Department of Heritage in Vientiane.

Their most recent expedition marked the fourth consecutive year of exploration at the site. Unfortunately, it coincided with the onset of travel restrictions following the COVID-19 outbreak. Nonetheless, findings from this expedition, conducted until the emergence of travel restrictions, were published on March 10 in the journal PLOS One.

Mysterious burials

Up to this point, the archaeological team has meticulously examined three of the numerous megalithic “jar sites” scattered throughout northern Laos. For their latest study, they honed in on the most renowned of these sites, known as Site 1. Situated just west of Phonsavan, it holds the distinction of being one of the 11 sites designated as a UNESCO World Heritage site. Site 1 spans over 60 acres (24 hectares) and boasts approximately 400 stone jars strewn across its landscape.

Dating the stone jars accurately poses a challenge. In 1935, the esteemed French archaeologist Madeleine Colani reported discovering human remains in some of them. However, modern archaeologists have yet to uncover datable human bones or teeth within the stone jars.

Nevertheless, evidence of three distinct types of burials has been uncovered at the jar sites: primary burials, where complete human skeletons were laid out; secondary burials, involving bundles of human bones interred; and burials within small ceramic jars, marked by distinct quartz boulders on the surface. These ceramic jar burials starkly contrast with the massive stone jars above ground, representing a burial practice relatively common in various parts of Asia at different periods.

Radiocarbon dating of the human remains found in the ceramic jars and other burial contexts suggests that the majority were interred between the ninth and 13th centuries—700 to 1,200 years ago—rendering them significantly younger than the stone jars themselves.

Sunrise at the Plain of Jars in northern Laos. The landscape is dotted with carved stone jars, some of them up to 10 feet (3 m) tall and estimated to be about 3,000 years old.

The latest research from the Plain of Jars combines the ages of some of the stone jars with the ages of some of the burials – most of the burials were found to be much younger than the stone jars.

Ancient glow

Shewan and O’Reilly’s team employed optically stimulated luminescence (OSL) on sediment samples taken from beneath some of the stone jars at Site 2, situated a few miles south of Site 1, to glean insights into the timeframe of their placement.

OSL measures the ionizing radiation absorbed by quartz grains within the sediments, enabling the calculation of when these grains were last exposed to sunlight.

“We obtained a date range of 1350 to 730 B.C. directly beneath one jar, and 860 to 350 B.C. beneath another,” remarked Shewan. “As we delve deeper into our analysis, we anticipate encountering a spectrum of dates.”

These dates would place the stone jars significantly earlier than the majority of the ground burials surrounding them. However, further research is essential to establish a correlation between the ages of the jars and the burials at the same sites, O’Reilly explained.

Determining whether different groups utilized the jar sites for mortuary customs at various periods or if later generations repurposed the ancient sites for burials remains an open question. “We’re unable to ascertain yet whether they shared cultural ties with the original jar makers,” O’Reilly noted.

During the latest expedition to the Plain of Jars, the team also scrutinized the ratios of lead and uranium isotopes within zircon crystals embedded in the rock of the stone jars. These isotopic compositions serve as a “chemical signature” indicative of their geological origin.