Back at the palace, they found that tiny keyhole in the wood

paneling — just where the plan suggested it would be. They had a key made so they could open the door – and they discovered a secret passageway 360 years old.

"To say we were surprised is an understatement — we really thought it had been walled-up forever after the war," Mark Collins, Parliament's estates historian, said in a statement. They knew such a passage had once existed, but believed that it had been filled in after the palace was bombed during World War II.

Behind the door was a small room, with hinges for a door that would have been more than 11 feet high and that would have opened into Westminster Hall.

It turns out to be a passageway with a rich history.