Tania Vassie was playing on the school oval when she looked up into the clear autumn sky to see a "strange, rounded, two-storey disc floating up above". The 13-year-old watched, transfixed, as the object moved through the sky erratically, dashing at speed from one direction to another. Then she ran, yelling, into the school. So began one of Australia's most intriguing mysteries. Tania wasn't the only one to see it. Scores of people watched from the school oval — some in amazement, some in terror — as an unidentified flying object swept across the skies above Westall High School and the adjacent primary school in Melbourne's south-east on April 6, 1966. Some saw up to three objects. It remains the biggest mass sighting of a UFO in Australian history. Many UFO enthusiasts are convinced it was an extra-terrestrial visitation. Others believe the answer to the mystery lies in a more Earth-bound explanation. Ken Stallard ... argues that, by definition, it was a UFO. "I saw what I saw and so did all my school friends, hundreds of us," he says. Not only did so many see the objects but multiple witnesses report that within 30 minutes, men in uniform arrived at a spot close to the school where some say they saw the craft descend. The school principal assembled the students and told them it was a weather balloon and not to talk about it. Some were ushered into a room where unidentified men told them to not to discuss what they'd seen.