Hundreds of megalithic stone jars, 1m-3m in height and weighing between 600kg and 6-7 tons, can be found strewn across the Phonsavan plain. Either standing straight or half-buried, scattered in isolation or together in small groups, they are said to be between 1 500 and 4 000 years old. Their purpose remains a mystery: were they used to store grain or to hold water during the dry season? Or were they containers in which to ferment rice to produce alcohol? Or perhaps used funerary urns? Scientists are still trying to find out.