immense underwater volcano, with a diameter of ; Pozzuoli is at its center. Approximately 12,000 years ago the last major eruption occurred, forming a smaller caldera inside the main caldera, with its centre where the town of Pozzuoli lies today. The Third Phlegraean Period, between 8,000 – 500 years ago. This is characterized by white pozzolana, the material that forms the majority of volcanos in the Fields. Broadly speaking, it can be said there was initial activity to the southwest in the zone of Bacoli and Baiae (10,000–8,000 years ago); intermediate activity in an area centred between Pozzuoli, Spaccata Mountain and Agnano (8,000–3,900 years ago); and more recent activity towards the west, which formed Lake Avernus and Monte Nuovo (New Mountain) (3,800–500 years ago). Volcanic deposits indicative of eruption have been dated by argon at 315,000, 205,000, 157,000 and 18,000 years ago. More recent history The caldera, which now is essentially at ground level, is accessible on foot. It contains many fumaroles, from which steam can be seen issuing, and over 150 pools (at the last count) of boiling mud. Several subsidiary cones and tuff craters lie within the caldera. One of these craters is filled by Lake Avernus. In 1538, an eight-day eruption in the area deposited enough material to create a new hill, Monte Nuovo. It has risen about from ground level since 1970. The volcanic island of Ischia suffered three destructive earthquakes in 1828, 1881 and 1883, the most destructive occurring in 1883. It had a magnitude of 4.2–5.2, but caused catastrophic shaking assigned XI (Extreme) on the MCS scale. Extreme damage was reported on the island and over 2,000 residents perished. At present, the Phlegraean Fields area comprises the Naples districts of Agnano and Fuorigrotta, the area of Pozzuoli, Bacoli, Monte di Procida, Quarto, the Phlegraean Islands (Ischia, Procida and Vivara). A 2009 journal article stated that inflation of the caldera centre near Pozzuoli might presage an eruptive event within decades. In 2012 the International Continental Scientific Drilling Program planned to drill below the earth's surface near Pompeii, in order to monitor the massive molten rock chamber below and provide early warning of any eruption. Local scientists are worried that such drilling could itself initiate an eruption or earthquake. In 2010 the Naples city council halted the drilling project. Programme scientists said the drilling was no different from industrial drilling in the area. The newly elected