Thirty-six barrels of gunpowder were discovered beneath Parliament during the 1605 Gunpowder Plot.
Officials found the barrels in a rented undercroft beneath the House of Lords late on 4 November 1605, shortly before the state opening of Parliament. Guy Fawkes was discovered guarding the explosives and was arrested during the search ordered by the authorities.
The conspirators intended the gunpowder to destroy the king, Parliament and much of the surrounding political leadership in one blast. The plotters had initially obtained a smaller amount, but their preparations eventually involved thirty-six barrels stored under the building.
The figure is sometimes confused with estimates of the explosion’s potential power or with modern descriptions of the amount of powder in the barrels. The discovery prevented the planned attack, and the event became the central historical episode commemorated by Bonfire Night fireworks and bonfires.