PIANOSA (anc. Planasia)  , an island of Italy, belonging to the province of Leghorn, and forming part of the communeof Marciana (Elba), from which it is 71 m . S.W . Pop . (1881), 774 . As its name indicates, it is quite flat, and the highest point is only 95 ft. above sea-level . Its area is 6 sq. m . Augustus banished to it his grandson, Agrippa Postumus, and some ruins of baths near the harbour still bear his name . It changed hands more than once in the wars between Pisa and Genoa in the 12th and 13th centuries; from 1390 it belonged to the prince of Piombino, but was depopulated in 1553 by the Turkish fleet, and only resettled at the beginning of the 19th century . In 1857 a penal colony was established here .