Hubby was the "lucky" one to open said jar; he did so in the shower, fearful it would explode (or, as Mr. Intensity is saying lately: KER-Splode!) I wish I'd taken a video of the process. It was like opening a 2-liter of Coke that had been dropped - quite fizzy. I thought for sure the stuff had gone bad.
But it tastes so good. Oh man. You gotta try it. I won't use any lids the next time. I talked with my parents and they remember their parents letting the sauerkraut ferment in the basement in a giant crock - without a lid.
If you want to try to make your own sauerkraut. It's really easy. If you can't find Nourishing Traditions at your library, here are two recipes. and one with pictures (thanks, Dad!) And it's good for you. Waaaay better than the store-bought processed and pasteurized stuff.