On 16 December 1857, the area around Padula, Italy, was devastated by the Great Neapolitan earthquake which caused 11,000 deaths. At the time it was the third largest known earthquake in the world and has been estimated to have been of magnitude 7.0 on the Moment magnitude scale. Mallet, with letters of support from Charles Lyell and Charles Darwin, petitioned the Royal Society of London and received a grant of £150 to go to Padula and record at first hand the devastation. The resulting report was presented to the Royal Society as the "Report on the Great Neapolitan Earthquake of 1857". It was a major scientific work and made great use of the then new research tool of photography to record the devastation caused by the earthquake. In 1862, he published the "Great Neapolitan Earthquake of 1857: The First Principles of Observational Seismology" in two volumes; he brought forward evidence to show that the depth below the Earth's surface, from where the impulse of the Neapolitan earthquake originated, was about 8–9 geographical miles.