The conspirators secured the use of part of a house, underneath the Prince's Chamber, occupied by John Whynniard. They drove, or were alleged to have driven, a tunnel, propped up by wooden piles, to the 12 foot thick foundation walls of the House of Lords.
Immediately underneath the Lords Chamber was a coal cellar leased to a man called Bright; it was here they intended to make a cavity. At the same time, they accumulated twenty kegs of powder in a house at Lambeth, planning to ferry them by boat at night to Westminster. However, Bright gave up his tenancy of the cellar on Lady Day (25 March) and the mining activities ceased when Thomas Percy stepped in and took a lease on the cellar itself.
Plans were also drawn up to secure the whereabouts of the heirs to the throne, Prince Charles and Princess Elizabeth, then children, and thus establish an ongoing friendly government to follow the explosion. During this time, the network of conspirators was of necessity much expanded; Warwickshire was the home territory of many of them. There is no doubt that the explosion was definitely planned for 5th November. Fawkes was to light the slow fuses; he was then to leave by boat and go to the Continent.
The plot was discovered, in the official version, through a mysterious anonymous letter to Lord Monteagle, a Catholic, warning him not to attend the State Opening. Whether the letter was genuine, from a fellow Catholic concerned about Monteagle's welfare, or whether it was a forgery and sent from within government circles, is uncertain.
In any event, on the 4th of November an initial search was made by Lord Monteagle and the Lord Chamberlain, Lord Suffolk. They discovered Fawkes and the wood and coal Percy had provided to cover the kegs of powder. Sir Thomas Knyvett, a retainer of the Court, and Justice for Westminster, then searched the cellar thoroughly with a group of men at midnight, found the gunpowder, and arrested Fawkes.