According to The Crown Chronicles:
The State Opening of Parliament begins a while before The Queen arrives at Westminster: it starts with the searching of the cellars. The Yeomen of the Guard, the oldest of the Royal bodyguards, search the cellars beneath the Houses of Parliament. This is a precaution to prevent a modern-day attempt at the Gunpowder Plot; in 1605, a Catholic plot hoped to assassinate James I and his family to put a Catholic on the Throne, by blowing up barrels of gunpowder in the cellars of Parliament.
A ‘hostage’ is also taken…
To read the entire article in The Crown Chronicles, please click here.