"Bradyseism is the gradual uplift (positive bradyseism) or descent (negative bradyseism) of part of the Earth's surface caused by the filling or emptying of an underground magma chamber and/or hydrothermal activity, particularly in volcanic calderas. More recently, between 1968 and 1972, the Campi Flegrei area suffered an episode of positive bradyseism and rose by 1.7 metres. Another rise of 1.8 metres occurred between 1982 and 1984"
https://en.wikipedia.org/wiki/Bradyseism"... showing that the Earth's crust could be subject to gradual movement without destructive earthquakes"
https://en.wikipedia.org/wiki/Macellum_of_PozzuoliSo: finally some pictures from me on this subject... (my visit was in january 2016)
1. Overview of the Macellum of Pozzuoli, with the sea and harbour in the background
(note the difference in altitude between the current level of the seawater and the Macellum)
2. The three pillars of the Macellum where the boreholes of the Lithophaga molluscs are visible, within the red square
(showing that the pillars were at one time, under seawater, without collapsing when they came up again)
3. Details of the three pillars with the boreholes
4. Details of one of the pillars with the boreholes
5. Last overview ; the water is rainwater
For the Phlegrean Fields, see
http://www.strahlen.org/forum/index.php/topic,566.0.html (for Active Members)