Exclave
Sandoval County, New Mexico: During World War II, Los Alamos County was created out of parts of Sandoval and Santa Fe Counties, for the convenience of the Manhattan Project.  That portion of Sandoval County which is within the San Ildefonso Indian Reservation, about 3 km², became an exclave bounded by Los Alamos County on the southwest, Santa Fe County on the east and Rio Arriba County on the north.