On the morning of 1 November 1755, during the All Saints' Day mass, an earthquake in the Atlantic estimated between magnitude 8.5 and 9.0 struck Lisbon. Forty minutes later tsunami waves entered the mouth of the Tagus and swept the harbour away; the fire that followed burned for five days. Estimates of the dead range from 30,000 to 50,000, and two-thirds of the city was left unusable.

The intellectual effect outlasted the destruction. That the catastrophe came on a religious feast morning, with the churches full, made it harder to defend the idea of the best of all possible worlds governed by a good God. Voltaire's Poem on the Lisbon Disaster (1756) took direct aim at Leibniz's optimism, and Candide is an extension of that argument. Rousseau's reply located responsibility not in divine order but in human decisions: it was people who had built seven-storey houses packed tightly along a fault. That objection is among the first statements of the modern understanding of disaster as the intersection of a natural event and a social choice.

The practical response points the same way. The king's minister, the Marquis of Pombal, sent a questionnaire to surviving clergy and governors asking how long the shaking lasted, from which direction it came, and how wells behaved afterwards — regarded as the first systematic documentation of an earthquake and placed at the beginning of seismology. In rebuilding the city, a timber cage frame known as the gaiola was used; absorbing seismic load by flexing, it is an early forerunner of modern earthquake codes.