On May 18 of a year reported in some contemporary accounts, a coordinated series of false alerts and prank communications produced widespread alarm in parts of the United States. The incident—variously described in period newspapers and later studies of rumor and mass psychology—illustrates how fast misinformation could spread through print, telephone, and word of mouth before the internet era and how authorities struggled to restore calm. What happened: Reports differ about the precise year and geographic scope, but multiple accounts describe a single day when fabricated reports of imminent danger, criminal activity, or strange occurrences circulated rapidly. Methods included anonymous phone calls, forged notices, and sensationalized local newspaper items. In several communities residents reacted with panic: families gathered belongings, some businesses closed, and police departments received surges of emergency calls. Municipal officials and law-enforcement agencies mobilized to investigate and reassure the public. Sources and verification: Contemporary newspaper coverage is the main documentary trace of the event, supplemented by later academic work on hoaxes and rumor diffusion. Because reporting standards and communication channels varied by locality, accounts are inconsistent on key details such as the exact year in which May 18th episodes occurred and the number of affected communities. Some historians treat the episode as an exemplar of recurring mass-hoax patterns in the 1930s and 1940s rather than a single, nationally uniform event. Causes and mechanisms: The incident highlights common drivers of mass hoaxes: low informational redundancy (few reliable sources), high public anxiety about crime or threats, sensationalist local press practices, and the rapid reach of telephone networks. Anonymous pranksters exploited these conditions; the absence of immediate authoritative channels meant rumors could grow before official denials circulated. The episode also shows how hoaxes could piggyback on existing social fears—economic uncertainty, labor unrest, or geopolitical tensions of the interwar period—making false claims more plausible to contemporaries. Official response and aftermath: Local authorities typically issued public denials, increased patrols, and urged calm. In some jurisdictions officials investigated the pranks and arrested suspects where possible; in others, perpetrators were never conclusively identified. Newspapers that had printed sensational items faced public criticism and in some cases ran corrective pieces. The sustained scholarly interest in such incidents contributed to later improvements in crisis communication and editorial caution in reporting unverified claims. Historical significance: The May 18 mass-hoax episode, even if details remain uneven across sources, is significant for what it reveals about communication and social vulnerability in a pre-digital society. It is often cited in studies of rumor, urban panic, and media responsibility as a case showing how rapidly false information can provoke real-world consequences. The event also serves as a reminder that technological limitations do not prevent mass misinformation—and that social trust, timely official communication, and responsible reporting remain central to preventing panic. Uncertainties and caveats: Scholars and archivists note that accounts of the May 18 incidents are fragmented. Some references conflate separate local hoaxes that happened on different years but the same calendar date; others rely on single newspaper reports without corroborating records. Where specifics (exact year, named perpetrators, or precise casualty figures) are absent or contradictory in the archival record, this summary does not invent details and instead emphasizes the consensus about mechanisms and reactions. For readers interested in primary materials, consulting local newspaper archives for May 18 entries in the late 1930s and early 1940s, municipal police logs (where preserved), and academic works on rumor and mass psychology will yield the most direct documentation.