At 9:40 in the morning of All Saints Day 1755 in Lisbon, a really big earthquake shook for several minutes; it was soon followed by a tsunami and many fires. Deaths are estimated to have been between 30 and 40 thousand.
The epicenter was southwards under the Atlantic, so there were an additional 10 thousand deaths in Morocco.
1755 was during the Enlightenment, so this was the first earthquake to be studied scientifically.
From Wikipedia:
"Eighty-five percent of Lisbon's buildings were destroyed, including famous palaces and libraries, as well as most examples of Portugal's distinctive 16th-century Manueline architecture. Several buildings that had suffered little earthquake damage were destroyed by the subsequent fire. The new Lisbon opera house (the "Ópera do Tejo"), opened seven months before, burned to the ground. The Royal Ribeira Palace, which stood just beside the Tagus river in the modern square Praça do Comércio, was destroyed by the earthquake and tsunami. Inside, the 70,000-volume royal library as well as hundreds of works of art, including paintings by Titian, Rubens, and Correggio, were lost. The royal archives disappeared together with detailed historical records of explorations by Vasco da Gama and other early navigators. The palace of Henrique de Meneses, 3rd Marquis of Louriçal, which housed 18,000 books, was also destroyed.[18] The earthquake damaged several major churches in Lisbon, namely Lisbon Cathedral, St Paul's Cathedral, Santa Catarina, São Vicente de Fora, and the Church of Nossa Senhora da Conceição Velha. The Royal Hospital of All Saints (the largest public hospital at the time) in the Rossio square was consumed by fire and hundreds of patients burned to death. The tomb of national hero Nuno Álvares Pereira was also lost. Visitors to Lisbon may still walk the ruins of the Carmo Convent, which were preserved to remind Lisboners of the destruction. Most of the documentation of the 1722 Algarve earthquake sent to Lisbon for archiving became lost after the fire that followed the 1755 earthquake."