Catacombs, underground burial sites, originated in first-century Rome and became more widespread in 17th-century Europe due to overcrowding and sanitation issues. Paris's catacombs, for instance, house the remains of over six million people within miles of quarry tunnels. While Paris is famous, similar crypt networks exist globally, such as those beneath Mexico's Templo Expiatorio and Washington D.C.'s Franciscan Monastery. Serbia's Petrovaradin Fortress hides a four-story network of military galleries with symbolic inscriptions. New York's Old St. Patrick's Cathedral catacombs inter early Catholic immigrants and notable figures like the Delmonico family. Naples, Italy, features the Catacombs of Saint Gaudiosus with ancient Christian art and later mummification practices, and the Fontanelle Cemetery Caves, a mass burial site with a unique "cult of the dead" tradition. Malta's St. Cataldus Catacombs showcase early Christian funerary rituals, and its larger complexes like St. Paul's and St. Agatha's date from the 3rd to 6th centuries. England's Warstone Lane Cemetery catacombs in Birmingham served as burial sites for the poor, while other examples include Portugal's Igreja de São Francisco and Rome's Sacconi Rossi Crypt for unclaimed bodies. These subterranean spaces reveal historical burial customs, religious practices, and societal conditions across different cultures and eras.