Determining epicentral locations and magnitudes of historical earthquakes has always been a foremost goal of historical seismology. From the 1980's onward such studies have produced abundant and important results in many countries and contexts, if not always entirely painlessly. Any historical seismologist knows the vexation of having to squeeze down an intricate investigation, full of brilliantly solved puzzles, into a couple of epicentral coordinates and an epicentral intensity value. More vexing still is finding a puzzle one is not able to solve and having to draw a conclusion from unsatisfactory data. Such an experience befell the writer some years ago, while working on the compilation of the Camassi and Stucchi (1997) Italian catalog, with the earthquake of 13 April 1558. Its memory rankled for a long time, until a clue of sorts came her way at a time favorable to a resumption of the quest to make sense of the previously enigmatic event. The historical investigation of the 13 April 1558 earthquake started again, from a completely new viewpoint, and about thirty previously unknown earthquake-related original documents were unearthed. These accounts are of tremendous value for understanding the 1558 earthquake, as this article will show by presenting and discussing them.