On 1st November 1755 the city of Lisbon in Portugal was hit by an earthquake, followed by fires and a tsunami. The city was almost completely destroyed by the effects and an estimated 30-40,000 people were killed.
The epicentre of the earthquake was not on the mainland, but about 120 miles away in the Atlantic Ocean. It had an estimated magnitude of about 8.7, making it one of the strongest earthquakes in history. In fact, many of the ones that are definitely stronger are those in modern times when the ability to measure the magnitude of earthquakes became possible.
The after-effects put a crimp in Portugal's colonial ambitions, as prior to that Portugal was a substantial naval and colonial power. It was also the first scientifically studied earthquake.
Image: By Jurema Oliveira (arquivos de Arte e História, Berlin) [Public domain], via Wikimedia Commons (commons.wikimedia.org/wiki/File:Lissabon-2.jpg)