Sinkholes on the outside of the heel may be caused by Keratosis Pilaris, tinea pedis, corns, and so on. 1. Keratosis Pilaris: It is a skin disease caused by bacterial infection, which is manifested by more crater-like pits on the outer side of the heel, i.e., sunken with a fossa. 2. Tinea pedis: it is a skin disease caused by fungal infection, which may lead to pimples, erosion between toes, and other symptoms, and may also appear on the outside of the heel depression with a nest. 3. Corns: a symptom caused by long-term friction and pressure on the feet leading to keratin hyperplasia, manifested as hard lumps on the soles of the feet and small holes in the depressions on the outside of the heels. It is recommended that the patient consult a doctor in time for a clear diagnosis and targeted treatment under the guidance of the doctor. Daily attention to personal hygiene, keep the heel local dry and fresh.