This event is widely considered to be one of the most catastrophic disasters in ancient Roman history. In 64 AD, a great fire broke out in Rome and raged for six days, destroying much of the city. The cause of the fire is still unknown, but it is believed to have been started
From Wikipedia
The Great Fire of Rome began on the evening of 18-19 July 64 AD. The fire started in the merchant shops around Rome's chariot stadium, Circus Maximus. After six days, the fire was brought under control, but before the damage could be assessed, the fire reignited and burned for another three days. In the aftermath of the fire, nearly three quarters of Rome had been destroyed.