Yup, that's the deal... .and it gets worse. Here's a bit about it.

"The herring are fermented in barrels for one to two months, then tinned where the fermentation continues. Half a year to a year later, gases have built up sufficiently for the once cylindrical tins to bulge into a more rounded shape.These unusual containers of surströmming can be found in supermarkets all over Sweden. However, certain airlines have banned the tins on their flights, considering the pressurized containers to be potentially dangerous.[2] Species of Haloanaerobium bacteria are responsible for the in-can ripening. These bacteria produce carbon dioxide and a number of compounds that account for the unique odor: ...

Yup, that's the deal... .and it gets worse. Here's a bit about it.

"The herring are fermented in barrels for one to two months, then tinned where the fermentation continues. Half a year to a year later, gases have built up sufficiently for the once cylindrical tins to bulge into a more rounded shape.These unusual containers of surströmming can be found in supermarkets all over Sweden. However, certain airlines have banned the tins on their flights, considering the pressurized containers to be potentially dangerous.[2] Species of Haloanaerobium bacteria are responsible for the in-can ripening. These bacteria produce carbon dioxide and a number of compounds that account for the unique odor: pungent (propionic acid), rotten-egg (hydrogen sulfide), rancid-butter (butyric acid), and vinegary (acetic acid).[3]

http://en.wikipedia.org/wiki/Surstr%C3%B6mming

Now, I think this should be at the absolute top of the disgusting foods list. I mean com'on! It's rotten fish!! Ewwww!!!! .