Guy Fawkes was arrested in the undercroft beneath the House of Lords.
The undercroft was a vaulted space below the medieval House of Lords at the Palace of Westminster. The plotters rented a cellar or undercroft beneath Parliament and stored barrels of gunpowder there, intending to destroy the Lords during the state opening.
Fawkes was found in the early hours of 5 November 1605 during a search ordered after an anonymous warning letter reached William Parker, 4th Baron Monteagle. He was carrying a lantern and gave the false name John Johnson when questioned.
The chamber is sometimes loosely described as a cellar, but “undercroft” is the precise historical term used in accounts of the discovery. The medieval undercroft itself survived the 1834 fire that destroyed much of the old Palace of Westminster, although the modern parliamentary complex is largely Victorian.