The Pentagon employs around 23,000 military and civilian people, as well as an additional 3,000 non-defense support staff. It has five sides, five above-ground floors, two basement levels, and five ring corridors per floor, totaling 17.5 miles (28.2 kilometers) of corridors. The building is also connected to other government buildings through a network of tunnels. There are actually 16 main levels inside the Pentagon. The first three are ground level, which includes entryways and parking lots. The next three are basement level, which contains utility rooms and storage. Then there are five main floors, which contain most of the office space. These include the Executive Office Floor, which houses offices for the Secretary of Defense, other top officials, and their staffs. There is also a Special Security Unit that works with the Secret Service to protect presidents and other high-profile individuals. On this floor you will also find the National Military Command Center, where officers watch over military operations worldwide from any one of several large conference rooms. Next comes the Congressional Floor, which consists of an atrium open to the public and 52 committee rooms where members of Congress can meet with staff and witnesses. At the end of this floor is the Library of Congress, where lawmakers can research issues before debating them on the House or Senate floor. The final main floor is called the Public Affairs Floor, which includes press conferences, television studios, and other facilities used by the Department of Defense and other agencies to communicate with the public.