Cerro Uritorco

Cerro Uritorco, a 1,979-metre granite peak overlooking the town of Capilla del Monte in Argentina's Córdoba Province, has been reported as a site of unidentified aerial phenomena since at least the 1960s, but its modern reputation was cemented by the case of Marcos Olmedo Marcos, a hiker who went missing on the mountain on 9 January 1986 and reappeared nine days later on a different slope, reportedly disoriented, dehydrated, and with no memory of where he had been — a case investigated at the time by Argentine military and civil defence personnel and never conclusively explained. The case made national news and transformed Capilla del Monte from a quiet resort town into a magnet for ufologists, with sightings reported by thousands of visitors during peak "wave" periods, particularly around the town's annual UFO festival.

Local folklore, drawing on both indigenous Comechingón tradition and more recent esoteric writing, holds that the mountain conceals an underground city called Erks, said to be inhabited by a technologically advanced civilisation that occasionally manifests as the lights seen in the night sky. Skeptical investigators, including members of Argentina's CASIOOA (a longstanding UFO research organisation that takes an explicitly scientific approach), have attributed the bulk of reported sightings to a combination of aircraft lights, the planet Venus, ball lightning associated with the area's quartz-rich granite (a phenomenon some researchers link to piezoelectric effects in stressed rock), and the psychological effects of visiting a site already primed with decades of UFO mythology.