Fermentation turns the can into a pressurized container
Surströmming is Baltic herring preserved by fermentation in brine. Microbes transform compounds in the fish, creating the distinctive fermented mixture; gas can also form as fermentation proceeds. Because the fish is sealed in a can, that gas has nowhere to go. Pressure builds inside, sometimes making the can bulge.
When the lid is pierced, the pressure difference drives gas and brine toward the opening. The escaping gas can carry droplets with it, turning a small puncture into a sudden, messy spray. The problem is not simply that the contents are liquid: it is the force pushing them out.
Water contains the burst, not the pressure
Opening the can underwater puts a layer of water between the opening and the surrounding air. As brine and bubbles emerge, the liquid disperses into the water rather than flying freely across a table. The water acts as a splash barrier, while the escaping gas can still rise in bubbles and reach the air.
That distinction matters: submerging the can does not remove the pressure or stop the gas from escaping. It mainly changes where the brine goes and how forcefully it can scatter. The result is a more controlled opening—not a sealed-off one—so the water may still carry some odor, but the table is less likely to take the brunt of the brine.
