ATSB report on Qantas depressurization/damage

[trof]

Basically, one of the O2 cylinders in the hold went ballistic and turned into a rocket.

Cylinder event
On the basis of the physical damage found with the aircraft forward cargo hold and
cabin, it was evident that the number-4 passenger oxygen cylinder had sustained a
failure that allowed a sudden and complete release of the pressurised contents. The
rupture and damage to the aircraft fuselage was consistent with being produced by
the energy associated with that release of pressure. Furthermore, it was evident that
as a result of the cylinder failure, the vessel had been propelled upward, through the
cabin floor and into the cabin space. Damage and impact witness marks found on
the structure and fittings around the R2 cabin door showed the trajectory of the
cylinder after the failure event.
http://www.atsb.gov.au/publications/investigation_reports/2008/AAIR/aair200804689.aspx