Natto is fermented soy beans. It has a very slimy texture due to the fermentation and is known for its strong smell. However, recently they are produced without any pungent smell, and are becoming widely appreciated. They were originally steamed, and wrapped in wara (Japanese hay) and left alone to ferment. It is the enzymes in the hay that actually helps create the natto, and they are extracted by the manufacturers to make it possible to ferment soy beans in containers.
Natto is widely known for its health benefits. It produces an enzyme called the nattokinase, which is a powerful compound to reduce blood clotting. Individuals who frequently consume it tend to have lower rates of cardiovascular disease.