Here’s what you’ll learn when you read this story:

• Archaeologists are taking a closer look inside the large stone pots dotting the famed—and mysterious—Plain of Jars in Laos.
• A research team excavated one of the largest jars, at over four feet tall, and found a mixture of human remains.
• The dating of the 37 different individuals packed into the jars rewrites the history of the sprawling oddity, which comprises a total of 2,100 jars.

In the highlands of central Laos, there are thousands of mysterious stone vessels whose origin and purpose have long been debated by archaeologists. More than 2,100 of these tubular-shaped megalithic stone jars, dating from between 500 B.C.E. and 500 C.E., dot a large swath of land known as the Plain of Jars in the center of the country’s Xiang Khoang Plateau. There have been lots of theories about their intended purposes, but the consensus has remained that they’re funerary objects of some sort.

However, millions of United States cluster bombs dropped during the Vietnam War still remain undetonated across Laos. So investigating those massive ancient stone jars—and by massive, we’re talking over four feet tall and six feet wide—is extremely tricky. Fortunately, a recent excavation of one of those jars has vastly expanded what we know about them.

In a new study published in Antiquity, a team of researchers managed to excavate one of the largest jars near the Laotian town of Phonsavan. Inside, they found a densely packed deposit of the remains of at least 37 people. Radiocarbon dating showed that the lifetimes of those 37 people spanned 270 years. “We determined that it was an example of secondary internment during the ninth and 12^th centuries, C.E., in which human remains were deposited after an initial period of decomposition elsewhere,” Nicholas Skopal, of James Cook University, said in a statement.

The more recent dating of the remains also challenges the long-held theory that the ancient jars date from Southeast Asia's Iron Age, repositioning the timeline of the Xieng Khouang Plateau in Laos, “one of Southeast Asia’s most enigmatic archaeological locations.”

“The number of individuals also suggests the jars were owned by family or extended family groups,” Skopal said. “They likely served as places where ancestral rites were performed over generations.”

The study of Jar 1 at Site 75 highlights the specialized function of these objects. The team believes that initial decomposition of remains happened in a different, smaller jar. Then, once the remains disintegrated, they were transferred to the larger storage container. The research team believes the large jars were likely just a stop along the path to another final resting place. This theory also explains why some of the jars are empty.

“The site highlights the critical value of exploring remote, undisturbed landscapes in Laos,” Skopal said. “The preservation encountered here offers an exceptional window into past mortuary practices, and indicates that many comparable sites may still exist, awaiting discovery.”

Adding to the question marks surrounding the thousands of jars were the 20 glass beads found within the analyzed example. Experts used chemical analysis to determine that the beads were crafted in South India and Mesopotamia, showing a previously unknown trade connection between folks living in the Laotian highlands and places beyond.

Along with the bones densely packed at the bottom of the jar, the team also recovered pottery shards, a small bell, and an iron knife, all common grave goods. Pavements of sandstone chips or limestone slabs were found, indicating the remains were packed down in layers.

“Although many scholars propose a mortuary function for the jars, definitive evidence is elusive and key questions regarding their purpose, age, and makers remain unresolved,” the authors wrote.

The dating offers a unique conundrum. The stone jars—the new study showed a mixture of stone was used to make Jar 1—were often crafted from stone quarried nearby, and may have been in place for centuries before remains were placed in them. Some sites feature just one jar and others have over 300, according to the Australian National University.

“Continued investigation of these landscapes,” Skopal said, “has the potential to fundamentally transform our understanding of the cultural and social dynamics that shaped the region.”