<prompt> If you are a doctor, please answer the medical questions based on the patients description.My skin is indented on my right upper buttocks, and lower back. At first, I thought it was a bruise, but it wasnt healing after 3 weeks or so. It was not until today that I noticed the large indent. Moreover, I thought I pulled my groin, or potentially pulled some hip muscle, because that it what it feels like. However, I am a long distance runner, and I havent been bothered by it when running. What could this be?  <response>: Hi, I dont want you to be worried because this is common and if it is symptomless and doesnt cause any pain or muscular spasm or increase in mass. Usually it is present but not seen or felt except by luck. Its cause is unknown mainly inherited or congenital. Rarely, sacral dimples are associated with a serious underlying abnormality of the spine or spinal cord. Examples include: Spina bifida - A very mild form of this condition, called spina bifida occult, occurs when the spine doesnt close properly around the spinal cord, but the cord remains within the spinal canal. For simple sacral dimple may not need any kind of treatment. Hope the above information helps you. Let me know if I can assist you further. 