The area of the caldera consists of 24 craters and volcanic edifices. Most of them lie under the Gulf of Naples. There are effusive gaseous manifestations in the Solfatara crater, which was believed in ancient Rome to be the home of Vulcan, the god of fire. The area features bradyseismic phenomena, which are most evident at the Macellum of Pozzuoli, misidentified as a temple of Serapis: bands of boreholes left by marine molluscs on marble columns show that the level of the site in relation to sea level has varied. Hydrothermal activity can still be observed at Lucrino, Agnano and the town of Pozzuoli.

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.

The Solfatara crater was accessible on foot until 2017 and contains many steam-emitting fumaroles and over 150 pools, at the last count, of boiling mud. Several subsidiary cones and tuff craters, one filled by Lake Avernus, lie within the caldera.

The magma underlying the Phlegraean Fields produces lavas of varying composition but generally rich in potassium. Trachyte is the most common eruptive product, unusually alkali-rich in some cases. Peralkaline phonolitic trachyte and latite have also been produced, and there is rare trachybasalt.

Three geological phases or periods are recognised and distinguished.

• First Phlegraean Period. It is thought that the eruption of the Archiflegreo volcano occurred about 39,280 ± 110 years (older estimate ~37,000 years) ago, erupting about 200 km (48 cu mi) of magma (500 km (120 cu mi) bulk volume) to produce the Campanian Ignimbrite eruption. Its Volcanic Explosivity Index (VEI) was 7 and it left a large part of eastern Europe covered in ash.
  "The dating of the Campanian Ignimbrite Eruption (CI) to ~37,000 calendar years B.P. draws attention to the coincidence of this volcanic catastrophe and the suite of coeval, Late Pleistocene biocultural changes that occurred within and outside the Mediterranean region. These included the Middle to Upper Paleolithic cultural transition and the replacement of Neanderthal populations by anatomically modern Homo sapiens, a subject of sustained debate. No less than 150 km (36 cu mi) of magma were extruded in this eruption (the CI eruption), traces of which can be detected in Greenland ice cores. As widespread discontinuities in archaeological sequences are observed at or after this eruption, a significant interference with ongoing human processes in Mediterranean Europe is hypothesized."

It is believed that the resulting ecological crisis wiped out both the last Neanderthal and the first Homo Sapiens populations of the early Upper Paleolithic. Modern humans then repopulated Europe from the east after the eruption and the ice age that took place from 38,000 to 36,000 BC.

The Phlegrean area is characterised by banks of piperno and pipernoid grey tuff at Camaldoli hill, as in the northern and western ridge of Mount Cumae; other referable deep products are those found at Monte di Procida, recognizable in the cliffs of its coast.

• Second Phlegraean Period, between 35,000 and 10,500 years ago. This is characterized by the Neapolitan yellow tuff that is the remains of an immense underwater volcano, with a diameter of c. 15 kilometres (9.3 mi); Pozzuoli is at its centre. 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.
• Third Phlegraean Period, between 8,000 and 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, Montagna Spaccata [Cleft 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.

In 1538, an eight-day eruption in the area deposited enough material to create a new hill, Monte Nuovo. It has risen about 2 m (7 ft) from ground level since 1970.

The volcanic island of Ischia suffered three destructive earthquakes in 1828, 1881, and the most destructive one in 1883, with a magnitude of 4.2–5.2 and causing catastrophic shaking assigned XI (Extreme) on the MCS scale. Extreme damage was reported on the island, and over 2,000 residents perished.

A 2009 journal article stated that deformation of the caldera centre near Pozzuoli might presage an eruptive event within decades. In 2012 the International Continental Scientific Drilling Program planned to drill 3.5 kilometres (11,000 feet) 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 were worried that drilling could precipitate an eruption or earthquake; programme scientists said it was no different from industrial drilling in the area. The drilling was halted in 2010, but later resumed. A Reuters article emphasized that the area could produce a "super volcano" that might kill millions.

A study from the Istituto Nazionale di Geofisica e Vulcanologia reported that the volcanic unrest of the Campi Flegrei caldera from January 2012 to June 2013 was characterised by rapid ground uplift of about 11 cm (4 in), with a peak rate of about 3 cm (1 in) per month during December 2012. It added that from 1985 to 2011 the dynamics of ground uplift were mostly linked to the caldera's hydrothermal system, and that this relation broke down in 2012. The driving mechanism of the ground uplift changed to periodical emplacement of magma within a flat sill-shaped magmatic reservoir about 3,000 m (9,843 ft) in depth, 500 m (1,640 ft) south from the port of Pozzuoli.

In December 2016, activity became so high that an eruption was feared. In May 2017 a new study by University College London and the Vesuvius Observatory published in Nature Communications concluded that an eruption might be closer than previously thought. The study found that the geographical unrest since the 1950s has a cumulative effect, causing a build-up of energy in the crust and making the volcano more susceptible to eruption.

On 21 August 2017 there was a magnitude 4 earthquake on the western edge of the Campi Flegrei area. Two people were killed and many more people injured in Casamicciola on the northern coast of the island of Ischia, which is south of the epicentre.

The reason for the seismicity and bulging ground is not known as of 2023. According to one model, called in a Scientific American article the shallow magma model, magma is pushing to break through the surface, making an explosive eruption with magma flow likely. In what has been termed the hot fluids model, steam and hot gases generated deeper underground are the cause. In this case activity could stop abruptly, or lead to a phreatic eruption of hot liquids, gases and rock fragments, rather than lava. While very dangerous, it is less so than a magma eruption. An answer to this question would help to estimate how likely this volcano, and other large volcanoes and supervolcanoes, are to erupt, and what warning signs to look out for.

A February 2020 status report indicated that earth uplift around Pozzuoli continued at steady rates, with a maximum average of 0.7 cm per month since July 2017. Gas emissions and fumarole temperatures did not change significantly. In May 2024 the ground was reported to be rising by 2 cm per month, with the rate increasing.

On April 26, 2020, a moderate earthquake swarm hit Campi Flegrei caldera, with about 34 earthquakes ranging between magnitude 0 and magnitude 3.1 centred around the port city of Pozzuoli. The strongest quake in the sequence was of magnitude 3.1, the strongest at the time since the last major period of unrest and rapid uplift in 1982-1984. However, no new fumaroles were reported.

Volcanic activity was reported in January 2022 to be increasing. In the year to September 2023 seismic activity had intensified, particularly in the later months. On 27 September 2023, a magnitude 4.2 earthquake—the strongest in 40 years—prompted the preparation of contingency plans to evacuate up to 360,000 people in the area. Despite a further 4.0 quake on 2 October, most volcanologists consider that a major eruption is not expected imminently.

The best-case scenario was deemed to be the activity ending, as happened after much activity on the 1980s. The worst would be an eruption like the one of 1538. A study by Italy's National Institute of Geophysics and Volcanology (INGV) and University College London (UCL) in June 2023 concluded that the volcano was edging towards "breaking point" and in an "extremely dangerous state". Nello Musumeci, Italian minister for civil protection, was to ask Neapolitan local officials for an "acceleration in the drafting of exodus plans in the event of an emergency"; he said that evacuation would only be carried out in the event of "extreme necessity". Mass evacuation plans were to be tested at the end of May 2024.

Intense seismic activity continues in 2024. On 20 May 2024, seismic activity intensified, with a swarm of 150 earthquakes in a few hours, one of magnitude 4.4 causing fear among the population of Pozzuoli and some cracks and minor damage to buildings. Schools and a prison were evacuated.

In respect of its 18th and 19th century role in the development of geoscience, not least volcanology, this locality was included by the International Union of Geological Sciences (IUGS) in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022.

Italian wine, both red and white, under the Campi Flegrei DOC appellation comes from this area. Grapes destined for DOC production must be harvested up to a maximum yield of 12 tonnes/hectare for red grape varieties, and 13 t/ha for white grape varieties. The finished wines need to be fermented to a minimum alcohol level of 11.5% for reds and 10.5% for whites. While most Campi Flegrei wines are blends, varietal wines can be made from individual varieties, provided the variety used comprises at least 90% of the blend and the wine is fermented to at least 12% alcohol for reds and 11% for whites.

Red Campi Flegrei is a blend of 50–70% Piedirosso, 10–30% Aglianico and/or Sciascinoso and up to 10% of other local (both red and white) grape varieties. The whites are composed of 50–70% Falanghina, 10–30% Biancolella and/or Coda di Volpe, with up to 30% of other local white grape varieties.

Campi Flegrei has had strategic and cultural importance.

• The area was the site of quarries for piperno stone, an ignimbrite (welded tuff), a stone that was used to build much of Naples; piperno "is probably the most important building stone of Naples, used over a time-span from at least the Roman age until the beginning of the 20th century." Piperno stone was used in the construction of Pompeii, along with other stone including Naples yellow tuff, foamy basalt, and limestone.
• The area was known to the ancient Greeks, who had a colony nearby at Cumae, the seat of the Cumaean Sibyl.
• The beach of Miliscola, in Bacoli, was the Roman military academy headquarters.
• Lake Avernus was believed to be an entrance to the underworld, and is portrayed as such in the Aeneid of Virgil. During the civil war between Octavian and Antony, Agrippa tried to turn the lake into a military port, the Portus Julius.
• Baiae, now partially submerged, was a fashionable coastal resort and was the site of summer villas of Julius Caesar, Nero, and Hadrian (who died there).
• In Pozzuoli is the Flavian Amphitheatre, the third-largest Italian amphitheatre after the Colosseum and the Capuan Amphitheatre.
• The Via Appia passed through the comune of Quarto, entirely built on an extinguished crater.
• The Cave of Dogs, a famous tourist attraction during the early modern period, is on the eastern side of the Fields.
• Europe's youngest mountain, Monte Nuovo, is here. A WWF oasis lies beside the enormous Astroni crater.
• The tombs of Agrippina the Elder and Scipio Africanus are here.
• At Baiae, now in the comune of Bacoli, the most ancient hot spring complex was built for the richest Romans. It included the largest ancient dome in the world before the construction of the Roman Pantheon.
• Astronomical broadcaster and writer Patrick Moore used to cite these Fields as an example of why the impact craters on the Moon must be of volcanic origin, which was thought to be the case until the 1960s.
• There is a theory that the Campanian Ignimbrite super-eruption around about 39,280 ± 110 years ago contributed to the extinction of the Neanderthals, based on evidence from Mezmaiskaya cave in the Caucasus Mountains of southern Russia.

• List of volcanoes in Italy
• Phlegra (mythology)
• Phlegraean Fields red zone

• Volcanism in the Campania Plain: Vesuvius, Campi Flegrei and Ignimbrites. Elsevier. 2006. ISBN 978-0-08-048166-1.
• Helmholtz Association of German Research Centres (16 Oct 2012). "An extremely brief reversal of the geomagnetic field, climate variability and a super volcano". Science X Network – via phys.org.

Wikimedia Commons has media related to Campi Flegrei.

• Phlegraean Fields
• Volcanological Excursion to Campi Flegrei
• Historical and Geological Introduction to the Neapolitan area
• Andrews, Robin George (14 November 2018). "Campi Flegrei Volcano's Ancient Cycle Seems to End in Large Eruption". The New York Times.