A giant helium balloon plunged 20 metres, injuring five people, at Ocean Park in Aberdeen yesterday afternoon. Among those injured was a mainland woman who was on the ground when he was hit on the head by the SkyStar balloon - measuring 22 metres in diameter - that fell outside its designated landing site. Two Indian tourists and two Hong Kong women were in the gondola of the balloon when it fell, and were treated for minor injuries. 'There was a big bang. We didn't know what happened and it fell. I was lucky I suffered only minor abrasions on my arms,' said one of the injured Hong Kong women, 60. She said it fell fast and landed on flower beds. The injured Indians left after they were bandaged at the scene, while the Hong Kong women were treated at Ruttonjee Hospital. The mainlander was treated at Queen Mary Hospital. Seven riders and an operator were on board the balloon, which had floated to a height 60 metres before it began its descent at 3.45pm. When it was about 20 metres from the ground, the operator noticed that the balloon was descending faster than its normal speed of 60cm per second. It then landed outside its 32-metre-diameter landing area. A mainland visitor said 'there was a hole of about three or four metres in the top of the balloon'. The three-year-old balloon is part of the SkyStar ride that began operating in 2007, the theme park said, adding that yesterday's incident was the first to occur at the attraction. The balloon, made in France, passed a weekly check by park management on Tuesday and an Electrical and Mechanical Services Department inspection on June 14. Ocean Park's executive director for engineering, George Tso Chi-keung, said it was not a sudden fall. 'There was definitely a leak, but we have to find out why this happened. There must have been a hole if it leaked,' he said. An investigation was under way. Last night, the electrical department demanded a report on the incident from the theme park within seven days. 'The department will conduct an independent and in-depth investigation to establish the cause of the incident,' its spokeswoman said. She said it was investigating why the balloon leaked and whether it was hit by strong winds. The park's website said the balloon, whose 1,100kg gondola could carry 29 riders and an operator, could rise up to 100 metres in the air.