In most cases, the dark circles under the eyes are blood vessels that can be seen through the skin. The skin around the eyelids (periorbital skin) is the thinnest skin in the body (around 0.5 mm thick compared with 2mm in other areas). Like varicose veins, dark circles under the eyes are usually an inherited trait. When blood passes through the large veins close to the surface of the skin, it can produce a bluish tint. The more transparent the skin—also an inherited trait—the darker the circles appear. In people with a deep-set bone structure, shadowing can also contribute to the dark color under the eyes.