Black hard mud in the navel can be caused by local dirt accumulation, skin debris and local inflammation and other factors. 1. Dirt accumulation: long-term not pay attention to the navel at the clean, due to the navel at the skin folds more physiological characteristics, sweat, grease, dust, etc. are easy to gather in the navel, can gradually form a black hard mud dirt. 2. Skin debris: human skin will shed some debris every day. The skin debris at the navel is easy to mix with clothes and dirt in the air, which accumulates in the folds of the navel skin and gradually forms black hard mud-like lumps. 3. Local inflammation: due to poor hygiene or trauma at the navel, can lead to bacterial infection in the navel, causing inflammatory symptoms, can appear in the navel in the black hard mud-like manifestations, but also accompanied by redness, swelling, pain and other uncomfortable symptoms. Black hard mud in the navel, should not be directly with the hands to pick, otherwise it will lead to skin breakage, easily lead to umbilical cord infection and other skin infections. It should be soaked in salt water and then removed with a cotton swab as much as possible.