Russell Crowe -- UFO Expert Calls BS On Spaceship Video

Russell Crowe did NOT see an alien spaceship outside his Australia office recently ... despite some pretty convincing footage he posted on the Internet ... so says a rep for the biggest UFO-debunking agency on the planet.

Russell posted the clip last week ... showing time-lapse video footage shot from the balcony outside his office in Woolloomooloo. Russell had wanted to capture video of fruit bats behind his office, but recorded some eerie-looking flying lights instead.

The aircraft in the clip is definitely weird-looking ... but Marc D'Antonio -- the chief video analyst for the Mutual UFO Network (MUFON) -- tells TMZ, it's definitely not other worldly.

D'Antonio says the aircraft is likely a plane or a helicopter instead -- the colors of the lights are consistent with common aircrafts, the linear light patterns can be attributed to the camera's long exposure time, and the other light patterns can be attributed to lens flare.

D'Antonio says misidentifying UFOs is common in long-exposure photography.

But here's where it gets creepy -- D'Antonio says real UFOs tend to exhibit strange flight patterns, unexpected color shifts, weird shapes, and extreme speeds. Yes, real UFOs. We asked D'Antonio to give us an example of a UFO he was unable to debunk ... and he sent us this.