The conspirators' original plan had been to tunnel to a
coal cellar under the Lords Chamber, from part of an
adjacent house occupied by John Whynniard. Whether
the tunnel was ever constructed is not known: in the
event, Thomas Percy was able to take up the lease on
the coal cellar in March 1605.
The detonation was to take place on State Opening day, when
the King, Lords, and Commons would all be in the same place.
20 kegs of gunpowder were stored across the river in Lambeth,
and ferried at night to Westminster, then hidden under firewood. Once preparations were complete, only Fawkes remained in London - intending to light slow fuses, then escape to the Continent by boat.