More recent investigations of the vertical movements have shown that the site is near the centre of the Campi Flegrei (Phlegraean Fields) caldera and has been subject to repeated "slow earthquakes" or bradyseism of this shallow caldera resulting in relatively slow subsidence over long periods, drowning the ruin, punctuated by periods of relatively rapid uplift that caused it to re-emerge. After a long subsidence through Roman times, there was a period of uplift in the Middle Ages around AD 700 to 800, then after more subsidence the land rose again from around 1500 up to the last eruption in 1538. The land again subsided gradually, then between 1969 and 1973 the land rose by about 1.7 metres (5.6 ft). Over the following decade there was a little subsidence, then between 1982 and 1994 there was uplift of almost 2 metres (6.6 ft). Concerns about risks of earthquake damage and possible eruption led to temporary evacuation of the city of Pozzuoli. Detailed measurements indicated that the caldera deformation formed a nearly circular lens centred near Pozzuoli. Various models have been produced to find mechanisms explaining this pattern.[13]
