Elephantine (ĕlˌəfăntĪˈnē) [key], island, SE Egypt, in the Nile below the First Cataract, near Aswan. In ancient times it was a military post guarding the southern frontier of Egypt. The Elephantine papyruses, which date from the 5th cent. B.C. and describe a colony of Jewish mercenaries, were found there. Surviving ruins are extensive. The ancient nilometer, built to gauge the water level of the Nile, was restored in 1870.