Guy Fawkes could have changed the face of London if his 1605 plot had not been foiled, explosion experts have said.
By Julianna Kettlewell
BBC News Online
His 2,500 kg of gunpowder could have caused chaos and devastation over a 490-metre radius, they have calculated.
Fawkes' planned blast was powerful enough to destroy Westminster Hall and the Abbey, with streets as far as Whitehall suffering damage, they say.
Another expert has said Guy Fawkes used 25 times the amount of explosive he really needed to destroy Parliament.
This begs the question: just how big a bang did Guy Fawkes intend to make?
Early in the morning of 5 November 1605, Guy Fawkes was discovered in a cellar under the House of Lords with 36 barrels of gunpowder and a 'slowmatch' to ignite the explosive.
He hoped to cause an explosion that would kill James I when he attended Parliament later that day.
The plan never came to fruition, and Fawkes, like the annual population of straw-stuffed effigies, faced a painful execution.
But if he had succeeded, explosion experts believe that King James' death might have been the thin end of the wedge, in terms of collateral damage.
As a bit of fun to mark Bonfire Night, the Institute of Physics asked researchers at the University of Wales to work out the size of blast Fawkes might have made.
They employed a neat little equation that calculates how much explosive was used in a detonation, by measuring the amount of damage done.
Of course, in Guy Fawkes' case, the amount of explosive is already known, and the damage is not - so the researchers did the equation backwards.
For the calculation, they assumed that gunpowder had roughly the same power as TNT, and that the explosion was outside rather than in a basement, which makes it, at best, a very rough estimate.
Fawkes was caught and executed
"We used the weight of explosive to work out how it would affect its surroundings," said Dr Geraint Thomas, head of explosion studies.
"From the pressure pulse generated by the explosion we can tell if windows are going to be smashed or if whole buildings will be demolished.
"From the amount of explosive that Guy Fawkes had, we can work out that if you were a third of a mile away you would have seen broken windows around you."
It is harder to say exactly what damage buildings would have suffered, because a detailed knowledge of their structure is needed, but it was likely to be great.
"There is a possibility that the Houses of Parliament and Westminster Hall would have been completely obliterated, although we can't know for sure," Catherine Gardner, of the University of Wales, told BBC News Online.
According to explosives expert Dr Sidney Alford, Catholic Fawkes used substantially more gunpowder than he needed to destroy Parliament.
In a report published in the New Civil Engineer, Dr Alford calculated that Fawkes and his fellow conspirators went for an overkill, filling the cellar beneath the House of Lords with 25 times the explosive necessary to bring the building down.
Guy Fawkes was no amateur in explosives. Before he became a professional plotter, he worked in the army, where his job was to pack gunpowder.
Therefore if he used 25 times too much gunpowder, maybe it was no accident.
David Reid, spokesman for the Institute of Physics, said: "This throws into question exactly how much damage Guy Fawkes intended to cause."