It will take some 10-15 minutes of aimless walk around the Plain before you will start getting this "eerie" feeling. Initially, you will notice the countless jars in variable sizes and conditions. On closer inspection, some of the jars have bullet marks from battles that took place in the Plain during the Vietnam era. Then your guide will point out the huge craters scarring the landscape, left over from relentless bombing of Eastern Laos by American bombers during the Vietnam war. Finally, the growing silence of the Plain, broken only by the constant whisper of the wind in everchanging tone and intensity and the depressing brown/beige colours of the landscape, blending with the constant mist on the horizon, plus an additional chill factor in the air, will create this "eerie" feeling.
Quick Tips:
There is no definite explanation of the manufacturing purpose of the jars, which are a few thousand years old. There have been numerous theories trying to explain their existence, usually across the ritual/religious spectrum, but no definite facts so far. Some Laotian scientists are doing extensive reasearch at the moment, following Western archeological training, and their findings are pointing towards a burial role for the jars; in other words, they offer a sarcophagus-type explanation.
Best Way To Get Around:
First you have to get to Phonsavan. Access from Luang Prabang or Vientienne is incredibly arduous--dare I say borderline masochistic--in a country with a road network that is really in bad shape, with very few kilometers asphalted. Your best bet is by air, and with very reasonable prices, Lao Airlines is a bargain.
A couple of Western embassies advise against the use of Lao Airlines, but I feel that risks are exacerbated and that your chance of an accident is much higher on the road.
From Phonsavan, you need a 4X4 van to take you to the Plain, and the Hotel Maly management can arrange this for you (see my journal: Accommodation to Phonsavan), as well as an English-speaking guide.