The Yonaguni Monument, discovered in 1987 by dive operator Kihachiro Aratake off Japan's Yonaguni Island, sits at number ten in Ancient Aliens' countdown of underwater mysteries that may evidence extraterrestrial contact. The massive stepped formation, lying just 60 feet below the surface, features what proponents describe as precise cuts, terraces, standing monoliths, and dolmen-like structures spanning a vast area. Japanese archaeologist Masaaki Kimura argues the presence of tools and engraved stones proves artificial construction, while Ancient Aliens theorists including Giorgio Tsoukalos suggest the monument could date to 20,000 years ago—when sea levels were lower during the last ice age. The episode connects the structure to Japanese imperial mythology, where emperors claimed descent from a dragon god's lineage, interpreted here as potential evidence of extraterrestrial ancestry.
Mainstream geologists, however, point to Yonaguni as a natural sandstone formation shaped by tectonic activity and erosion, noting that the region's geology naturally produces angular, stepped patterns. The "too precise" cuts cited by believers align with known fracture patterns in stratified rock. While the 10,000-20,000 year dating remains speculative and tied to sea level assumptions rather than direct evidence, the episode offers genuine intrigue for skeptics: Yonaguni's geometric appearance is genuinely striking, the site remains understudied compared to land-based archaeology, and legitimate questions persist about coastal civilizations lost to post-ice age flooding—even if extraterrestrial intervention isn't the necessary explanation.