Eruption of Mount Vesuvius

The eruption of Mount Vesuvius, a volcano located on the west coast of Italy, (PROSE: A History of Humankind) occurred on 24 August 79, destroying the Roman city of Pompeii, (TV: The Fires of Pompeii) and burying the nearby town of Herculaneum in ash, along with several other local Roman settlements. (PROSE: A History of Humankind)

Prelude
A group of Pyroviles landed in the region of Pompeii and slept for thousands of years until they were awakened by an earthquake in 62 AD. They set in motion a plan to xenoform the Earth into a planet of fire, to become their new home following the disappearance of their homeworld of Pyrovillia. (TV: The Fires of Pompeii) As the Tenth Doctor would discover, Pyrovillia was among the planets which, as part of the Planetary Relocation Incident, were taken out of time and space by a magnetron used by the New Dalek Empire to form an engine for the reality bomb. (TV: The Stolen Earth/Journey's End)

According to rumours in March 64 AD, Seneca had died in Pompeii the month before. (PROSE: Byzantium!)

Volcano Day
The rocklike Pyroviles used a hollowed-out chamber in the mountain as part of their plan to terraform the Earth into a rocky wasteland to replace their lost homeworld, Pyrovillia. They used the energy from the volcano to power their operations, which cancelled the eruption. This presented the Doctor with a moral dilemma: by stopping the Pyroviles he would trigger the eruption, saving the world but at the cost of Pompeii and 20'000 of its inhabitants. With the support of his companion, Donna Noble, he reversed the process and Vesuvius vengefully erupted and destroyed the Pyroviles. The explosion was as powerful as twenty-four nuclear bombs. The force of the blast cracked open a rift in time which lasted for a second. Its effects rippled backward in time, allowing the people of Pompeii to see the future. The eruption also destroyed the Pyroviles. Although almost all of the residents of Pompeii perished, one family was saved by the Doctor. This was a fixed point in time, though the Doctor was unaware he had caused it at first. (TV: The Fires of Pompeii)

The Daleks once stole the Mark One model of a time machine called the Time-Conveyor, which was built by Peter and David's uncle. They hoped to reverse-engineer their own time machine from it. Peter and David pursued them through time in the Mark Two. They finally recovered it in Pompeii during the eruption of Vesuvius, having defeated the Daleks one-by-one along the way. (PROSE: Timechase)

Captain Jack Harkness claimed to have used the eruption as part of one of his cons against the Time Agency on multiple occasions, though he would never stay for "Volcano Day". (TV: The Doctor Dances)

Aftermath and legacy
Upon returning to Chiswick just prior to the Sontaran invasion of Earth, Donna thought of Pompeii amongst other recent experiences. (TV: The Sontaran Stratagem) Later, whilst looking into the disappearance of Earth among other planets at the Shadow Proclamation with the Doctor, Donna brought up the mention of Pyrovillia's disappearance in Pompeii. Though the Judoon deemed this an irrelevant cold case, the Doctor realised that the current Planetary Relocation Incident involved the relocation of planets from time as well as space to the Medusa Cascade. (TV: The Stolen Earth) After defeating the New Dalek Empire, the DoctorDonna saw that Pyrovillia and all the other planets were returned from the Cascade. (TV: Journey's End)

The Doctor told Donna that he was not going to forget Caecilius' face in a hurry, as he considered it to be a good, "strong" face. (PROSE: The Fires of Pompeii) Indeed, the Doctor later took on the face of Caecilius when he regenerated into his twelfth incarnation. (TV: The Time of the Doctor) Although unsure why his new face appeared familiar at first (TV: Deep Breath), he came to realise that it was to remind himself that his purpose as the Doctor was to always save people — no matter how impossible or "wrong" it might seem. (TV: The Girl Who Died)

Caecilius's legacy lived on through his descendants, Evie and Maxine, all the way to the year 2020, where his distant relatives (who bore a striking resemblance to his past wife and daughter) conversed via video call during the COVID-19 pandemic. (WC: The Descendants of Pompeii)

In 1941, Captain Jack Harkness admitted his part in Pompeii to Ninth Doctor and Rose Tyler, the former of whom referred to the then ongoing Empty Child Syndrome outbreak as "volcano day". (TV: The Doctor Dances)

The Seventh Doctor's companion Melanie Bush made a replica of Mount Vesuvius out of papier mâchè while in primary school. (AUDIO: The Fires of Vulcan)

Alternate timelines
In an alternative timeline, Vesuvius took much longer to erupt and the people of Pompeii evacuated in their boats. (PROSE: The Algebra of Ice)