An Underwood cleft or septum is a fin-shaped projection of bone that sometimes exists in the maxillary sinus on or near the floor of the sinus cavity and is often called a maxillary sinus septum or Underwood’s septa. Its name is derived from Arthur S. Underwood, the anatomist at King’s College in London who first discovered these bones in 1910. An Underwood septum is of special interest to dental professionals when performing surgical procedures designed to elevate the sinus floor, because it increases the likelihood of potentially serious complications like tearing of the Schneiderian membrane, or the thin lining of the maxillary sinus cavity. As many as 32% of patients have been reported to have an Underwood’s septa, and they can often be seen on dental x-rays without the need for additional imaging. Two types of septa exist; primary septa were originally described by Underwood and secondary septa can form after tooth loss.