POPULONIA. - Ancient Etruscan city (Pupluna) and ancient diocese on the promontory of Piombino near Porto Baratti; it was a Roman municipium but already in decline by the end of the Republic and nearly abandoned according to Rutilius Namatianus.
It was occupied by the Lombards and donated by Charlemagne to Hadrian I, but was destroyed in the 9th century and the see transferred to Cornia during the pontificates of Nicholas I and Stephen V (886). Its bishops were first directly subject to the Holy See, then Innocent II in 1133 united the see of P. with the church of Pisa; subsequently the see of Massa Marittima was established (v.). Among its bishops are remembered: Atellus, present at the Synod of 501; Cerbonius, mentioned in 591 by St. Gregory (Dial., III, 11); Paul, who was legate of Nicholas I in Bulgaria; Alexander II in 1072 granted privileges, Gregory VII in 1074 gave jurisdiction over the island of Elba and sent as his legate to Sardinia the bishop William.