The common causes of a line under the eyes include blood vessels, tear ducts, pigmentation, age, and late nights. 1. Due to hyperpigmentation: It is related to the precipitation of melanin, if melanin precipitates around the eye bags or eye circles, a line will be formed. 2. Tear trough: Tear trough will generally grow with age, there will be the phenomenon of tear trough depression, at this time will form a line under the eye. 3. Due to blood vessels: human facial skin is relatively thin, white people can clearly see the skin under the eyes blood vessels, this is a normal physiological phenomenon. 4. Due to age: with the increase of age, collagen will be lost, so that the skin wrinkles, there will be a line in the eye. 5. caused by staying up late: often stay up late will make the skin loose, sagging skin, this phenomenon. It is recommended to do an eye examination under the guidance of a doctor to treat the symptoms.