Early Modern Letters Online have released a catalogue of the correspondence of the Jesuit polymath and scholar Athanasius Kircher (1602-1680), containing details of 2,741 letters. It can be viewed here.

ATHANASIUS KIRCHER. FRONTISPIECE, MUNDUS SUBTERRANEUS (1664). (BODLEIAN LIBRARY, UNIVERSITY OF OXFORD, DOUCE K 148; IMAGE: MARC KOLAKOWSKI)