A 27-year-old woman presented with an acute, tender, geographic lesion on her left shin that developed after contact with a brain coral while scuba diving. Photographs obtained two hours after the incident reveal a well-demarcated, geographic, erythematous plaque with a serpiginous and cerebriform pattern at the site of contact, resembling the outermost surface contour of brain coral. The plaque resolved spontaneously over a three-week period. The biology of corals and potential biological features that lead to cutaneous eruptions are reviewed.