The nearby islets of Gavutu and Tanambogo housed the Japanese seaplane base as well as 536 Japanese naval personnel from the Yokohama Air Group and 3rd Kure Special Naval Landing Force and Korean and Japanese civilian technicians and laborers from the 14th Construction Unit. The two islets were basically mounds of coral—both about high—and connected to each other by a -long causeway. The hills on Gavutu and Tanambogo were called Hills 148 and 121 respectively by the Americans because of their height in feet. The Japanese on both islets were well entrenched in bunkers and caves constructed on and in the two hills. Also, the two islets were mutually supportive since each was in machine gun range of the other. The U.S. mistakenly believed the islets were garrisoned by only 200 naval troops and construction workers.