Aviation investigators say they do not yet know what caused a hole to open in a Qantas jumbo jet that was forced to make an emergency landing in the Philippines.

Australian aviation investigators began examining the U.S.-made Boeing plane in Manila Saturday.

None of the 346 passengers and 19 crew members were hurt during what was described as an "explosive decompression" over the Pacific Friday. The passengers have since returned safely to Melbourne, Australia.

Australian media reports say the plane had a history of corrosion. But Peter Gibson of Australia's Civil Aviation Safety Authority discounted that as a cause.  And Qantas chief executive Geoff Dixon says the corrosion found earlier this year was not near the part of the plane that ruptured.

Investigators have also discounted the possibility that a bomb caused the hole.  But Australian investigators are looking at the possibility that a pressurized container exploded in the luggage hold, or a fuselage panel tore loose from the plane.

The 17-year-old Qantas Airways Boeing 747 was heading to Melbourne from London via Hong Kong when passengers heard a loud bang.

The pilot quickly took the plane down more than 6,000 meters to restore cabin pressure.  Video shot by a passenger showed everyone inside the cabin acting calmly as oxygen masks dangled from the ceiling.

Some information for this report was provided by AFP and AP.