Populonia was besieged by Sulla, and in Strabo's time was already declining; later it suffered at the hands of Totila, King of the Lombards, and in 817 of a Byzantine fleet.

After this, the bishops of Populonia abandoned the town, which was destroyed in the 9th century, and moved their seat to Castrum Corniae.

[7] The Second Vatican Council (1962–1965), in order to ensure that all Catholics received proper spiritual attention, decreed the reorganization of the diocesan structure of Italy and the consolidation of small and struggling dioceses.

[8] This applied to Populonia, which had a tiny population but was part of the name of the diocese, while the much larger city of Piombino, which was a civil administrative center, had no recognition.

[12] The cathedral of Massa, begun in the first years of the 11th century, is dedicated to Saint Cerbonius, one of its earliest bishops, who was named protector of the city.

[13] The cathedral is served and administered by a Chapter, composed of two dignities (the Archpriest and the Provost) and ten Canons.

(1394–1404) held a diocesan synod on 20 May 1396, at which, having considered the effects of the wars and famine, it was decided to reduce the decima (tithe).

[22] On 18–20 April 1723, Bishop Eusebio Ciani (1719–1770) presided over a diocesan synod, held in the cathedral at Massa.