HONGEO (홍어) is raw, rotten skate fish (similar to a stingray). The skate fish has no bladder, so it urinates through its skin, giving the meat an ammonia smell and flavor. A long time ago fisherman discovered (by accident) that even though their skate fish had become rotten, it was still edible. Not only that, but many people actually liked the flavor, so HONGEO began to become popular.
It wasn't the most delicious food I tried during my trip to Korea, but to quote my friend DINGULSIN, "It was edible."
I tried it together with my friend DINGULSIN (뒹굴신). Check out her channel here: https://goo.gl/HFpX48