Per the trusted Wikipedia source, Hakarl is fermented shark. The reason it is fermented is because the shark itself is poisonous when fresh due to a high content of uric acid and trimethylamine oxide (aka urine).
The reason I know this is we returned from Iceland about 3 hours ago and some people in Iceland consider Hakarl a food.
As background for the uninitiated, Hákarl is traditionally prepared by gutting and beheading a Greenland or basking shark and placing it in a shallow hole dug in gravelly-sand, with the now-cleaned cavity resting on a slight hill. The shark is then covered with sand and gravel, and stones are then placed on top of the sand in order to press the shark. The fluids from the shark are in this way pressed out of the body. The shark ferments for 6-12 weeks depending on the season in this fashion.
Following this curing period, the shark is then cut into strips and hung to dry for several months. During this drying period a brown crust will develop, which is removed prior to cutting the shark into small pieces and serving. The modern method is just to press the shark’s meat in a large drained plastic container.
First-timers are sometimes advised to pinch their nose while taking the first bite as the smell is much stronger than the taste. It is often eaten with a shot of the local spirit, a type of akvavit. Eating hákarl is often associated with hardiness and strength.
Those new to it will usually gag involuntarily on the first attempt to eat it due to the high ammonia content.
We had not read this last part before our trip, but a picture (or series of pictures) is worth a thousand words.