The earliest works of 19th-century Serbian language reformer Vuk Stefanović Karadžić were published in Vienna by a printing house of Armenian Mechitarists. The Mechitarists also published the works of other Serb authors. In total, they printed 37 books and brochures, including "The Mountain Wreath" by Montenegrin Prince-Bishop Petar II Petrović-Njegoš. A "khachkar" (Armenian cross-stone), high and made of volcanic rock, stands at the entrance of the Church of the Archangel Gabriel in the Belgrade municipality of Zemun. The monument was erected in 1993, and it commemorates the Serbian pilots who perished in a plane crash in 1988 while transporting humanitarian aid to Armenia after the country was struck by a catastrophic earthquake.