Kircher had over 760 correspondents, including scientists, physicians, Jesuit missionaries, two Holy Roman Emperors, popes, and potentates throughout the globe. The subjects discussed in his voluminous correspondence cover the entire range of his interests. Letters sent to Kircher were commonly accompanied by curious natural objects or artefacts for Kircher's expanding collection in Rome and reports of astronomical observations or experiments performed by the global network of Jesuit missionaries. This immense correspondence, which remained unpublished for three centuries, constitutes a hugely important resource for the study of early modern Europe.