The fifteen-kilometre-wide stretch of grassy meadows and low rolling hills around Phonsavan takes its name from the clusters of chest-high urns found here. Scattered across the Plain of Jars and on the hills beyond, the ancient jars, which are thought to be around two thousand years old, testify to the fact that Xiang Khouang province, with its access to key regional trade routes, its wide, flat spaces and temperate climate, has been considered prime real estate in Southeast Asia for centuries. The largest jars measure 2m in height and weigh as much as ten tonnes.

Little is known about the Iron-Age megalithic civilization that created the jars, but in the 1930s, bronze and iron tools as well as coloured glass beads, bronze bracelets and cowrie shells were found at the sites, leading to the theory that they were funerary urns, originally holding cremated remains. More recent discoveries have also revealed underground burial chambers.