The Pentagon is the headquarters of the United States Department of Defense, located in Washington, D.C. It's one of the largest office buildings in the world, with an outside perimeter almost a mile long and containing 17.5 miles of corridors which are walked by some 26,000 employees and personnel. But it's also one of the most efficiently-designed -- any two points within the building are no more than a seven-minute walk apart.

It was conceived in mid-July 1941, and built with astonishing speed. Groundbreaking took place on September 11 of that year, and construction was completed just sixteen months later. Its size allowed it to replace a total of seventeen pre-existing War Department buildings and connect them far more efficiently. It was built with fifteen miles of pneumatic tubes for mail delivery, which are actually still used today.

Of course, it's secure. But no security is perfect, and the DoD knows that they're one of the biggest targets in D.C. or the country. The Pentagon Building Security and Emergency Procedures Guide is a twenty-two page handout briefing Department employees on subjects from rules regarding visitors and security passes, to how to recognize a possible mail bomb that slipped through screening, to how to deal with a possible hostage situation in your area. The last page includes a checklist of things to ask a caller if your phone rings with a bomb threat.

I'd hate to be the guy responsible for maintaining their firewall.