What is surströmming made of? And why does the surströmming smell so bad?

Small Baltic herring are caught in the spring, salted and left to ferment at leisure before being stuffed in a tin about a month before it hits the tables and shops. The fermentation process continues in the tin; ‘souring’ as the Swedes refer to it, and results in a bulging tin of fermented herring or surströmming. The aroma is pungent, and the taste is rounded yet piquant with a distinct acidity. The fermentation originates from a lactic acid enzyme in the fishes' spine.