A mysterious green fireball filmed flying over parts of eastern Japan in the early hours of Monday have left many people wondering what it actually was.

One Twitter user assumed it was an extraterrestrial visitor after the object was seen around 5:30 a.m. Monday, while another joked it was a plasma fireball launched by Gamera, a giant monster from a popular special effects sci-fi film series of the same era and type as Godzilla.

Disappointing as it may be, Hitoshi Yamaoka, a public relations chief at the National Astronomical Observatory of Japan in Tokyo, told The Japan Times that the Halloween mystery was most likely a bolide, or an extremely bright meteor that explodes in the atmosphere, a phenomenon that is not unusual.