Explosions triggered by molten aircraft metal reacting with water from sprinkler systems may have felled the Twin Towers after the 9/11 attacks, according to a new theory.

Dr Simensen calculated that the temperature of alluminium alloy in the aircraft hulls could have reached 750 degrees Celsius...

Just before the two World Trade Centre skyscrapers in New York collapsed on September 11 2001, powerful blasts were heard within the buildings.

A leading materials scientist says they could be the key to the dramatic conclusion of the terrorist attacks.

Over-heated steel beams have been blamed for the towers suddenly crashing to the ground after they were hit by two passenger jets.

But Norwegian expert Christian Simensen has another theory. He believes powerful explosions caused by a chemical reaction between molten aluminium from the aircraft and water ripped out the buildings’ internal structure.

“Both scientific experiments and 250 reported disasters suffered by the aluminium industry have shown that the combination of molten aluminium and water releases enormous explosions,” said Dr Simensen, from the SINTEF institute based in Trondheim, Norway.

Almost 3,000 people died after al Qaida terrorists flew two fuel-laden passenger jets into the World Trade Centre in Manhattan.

The impacts triggered massive explosions and fires, but the subsequent collapse of each tower came as a shock and surprise to those watching the disaster unfold.

Explosions heard just before the buildings fell have led to claims that explosives were set off inside the towers.

Dr Simensen believes after crashing into the skyscrapers the two jets would have been trapped within an insulating layer of debris.

As a result, the aircraft hulls rather than the buildings absorbed most of the heat from the burning aviation fuel.

Molten aluminium from the jets, flowing down through staircases and gaps in the floors, reacted with water from emergency sprinklers on the lower levels, it is claimed.

“I regard it as extremely likely that it was these explosions that made the skyscrapers collapse by tearing out part of the internal structure, and that this caused the uppermost floors of the buildings to fall and crush the lower parts,” said Dr Simensen, who presented his theory at an international materials conference in San Diego, California.

“In other words, I believe that these were the explosions that were heard by people in the vicinity and that have since given life to the conspiracy theories that explosives had been placed in the skyscrapers.”

The Boeing 767 aircraft would have together carried 30 tonnes of aluminium, he said.

Dr Simensen calculated that the temperature of alluminium alloy in the aircraft hulls could have reached 750C, enough to make it as liquid as water.

He told the Minerals, Metals and Materials Society: “All the floors in the Twin Towers were equipped with sprinkler systems. All the water above the hot aircraft bodies must have turned to steam. If my theory is correct, tonnes of aluminium ran down through the towers, where the smelt came into contact with a few hundred litres of water. From other disasters and experiments carried out by the aluminium industry, we know that reactions of this sort lead to violent explosions.

“The aluminium would immediately react with the water, with the result of a local rise in temperature of several hundred degrees, in addition to the explosions that were due to the fact that these reactions release hydrogen. Such reactions are particularly powerful when rust or other catalysts are present, which can raise the temperature to more than 1500C.”

The dangers of combining molten aluminium and water are well known to the aluminium industry, Dr Simensen added. In one experiment, in which 20 kilos of aluminium smelt were allowed to react with 20 kilos of water containing some rust, the resulting explosion destroyed an entire laboratory and left a crater 30 metres in diameter.

“Aluminium-water explosions are like dynamite explosions,” Dr Simensen said. “They were probably powerful enough to blow out an entire section of each building. The top section would than fall down on top of the sections that remained below, and the sheer weight of the top floors would be enough to crush the lower part of the building.”