USO (Unidentified Submerged Object)

A USO — Unidentified Submerged Object — is an unknown observed entering, leaving, hovering over, or moving beneath water, the aquatic counterpart to a UFO. The label follows the same "unidentified, not extraterrestrial" logic as its airborne parent term and dates to the 1960s, when investigators needed a way to file reports that didn't fit a purely aerial category. The anchor case is Shag Harbour, Nova Scotia, in October 1967, when multiple witnesses and RCMP officers watched a large object descend into the harbour, and Canadian military divers searched the site for days afterward without finding wreckage. The 2013 Aguadilla, Puerto Rico infrared video — an object filmed by a Customs and Border Protection aircraft appearing to submerge off the coast without a splash — is the modern flagship USO case and a centerpiece of the "five observables" discussion around trans-medium travel. USOs matter to the wider debate because water crossings are one of the harder things to fake or misidentify: a genuine air-to-water transition without deceleration is not something conventional aircraft or drones do, which is exactly why AARO's "all-domain" mandate explicitly folds USOs in alongside airborne and orbital reports rather than treating water sightings as a separate, lesser file.