I had heard of the Plain of Jars before I got to Laos, although I have no idea how. A chance encounter with a Canadian at a restaurant convinced me that it would be worth going out of my way to get to it, and she was indeed correct. The jars vary in size, apparently to fit the size of the sandstone boulder they were carved from. There are three large sites and several small ones littered with these things, and they have no idea what they're for. People have fielded theories (funerary, fermenting lao-lao), but they just don't have enough information. Most of them used to have lids, but over the years the lids were taken by locals for flour grindstones, for foundation stones for houses, and to break in pieces for grindstones for knives. A few lids are left, and no more should be taken as the government has enclosed the sites. I got to all three major sites in the day I was in Phonesavan, and it was very peaceful and very interesting.
There's a downside to being in Laos (or Vietnam, and to a lesser extent Cambodia). That's "UXO" or UneXploded Ordinance. They live with the violent ghost of the Vietnam war, and it still claims 40 to 60 people a year (mostly children playing in the fields) and probably a good number of water buffalo as well. They're trying to clean it up, but don't have a lot of money to put toward sweeping an entire country for mines. As a Canadian, I can't really conceive of living under these conditions. The Plain of Jars sites are very carefully marked out to indicate which areas have been cleared and which haven't. Despite this spectre hanging over the fields, it was a very peaceful and beautiful way to spend a day.