Pentagon

The Pentagon was a building housing the United States Department of Defense. Bill and Nardole visited it in the Monks' virtual reality. (TV: Extremis)

History
Just prior to the Dalek invasion of Earth in 2009, the Pentagon issued an emergency report as two hundred Dalek flying saucers approached the planet. (TV: The Stolen Earth)

During the Year of the Slow Invasion, Shakri cubes caused a systems breach at the Pentagon. (TV: The Power of Three)

It was one of the organisations that Tanya Adeola attempted to hack as a hobby. (PROSE: The Stone House)

Behind the scenes
Pentagon is the building housing the US Department of the Defense headquarters. The name of the building is often colloquially used for the organisation itself.