(I am a Marine, but I hold the Tomb Guard in the Highest Esteem! - 3rd U.S. Infantry Regiment)
It is considered one of the highest honors to serve as a Sentinel at the Tomb of the Unknowns. Fewer than 20 percent of all volunteers are accepted for training and of those only a fraction pass training to become full-fledged Tomb Guards. This attrition rate has made the Tomb of the Unknown Soldier Guard Identification Badge the second least-awarded decoration of the United States Military (the first being the Army Astronaut Badge).
The soldier "walking the mat" does not wear rank insignia, so as not to outrank the Unknowns, whatever their ranks may have been. Non-commissioned officers (usually the Relief Commander and Assistant Relief Commanders), do wear insignia of their rank when changing the guard only. They have a separate uniform (without rank) that is worn when they actually guard the Unknowns or are "Posted".
The duties of the sentinels are not purely ceremonial. The sentinels will confront people who cross the barriers at the tomb, or are disrespectful or loud.
Over the years there have been several different types of weapons used by the Tomb Guards. The changes in weapons reflect the changes in the Army, including M1903 Springfield rifle, M1 Garand and M14 rifles, M1911 .45 ACP and M9 9mm Beretta pistols. Tomb Guards currently carry M14 rifles, which are unloaded and affixed to ceremonial rifle stocks (hand-made by Tomb Guards). These rifles are cleaned daily and kept ready for use at all times.
There is a meticulous routine which the guard follows when watching over the graves:
The soldier walks 21 steps across the Tomb. This alludes to the twenty-one gun salute, which is the highest honor given to any military or foreign dignitary in America. His weapon is always on the shoulder opposite the Tomb (i.e., on the side of the gallery watching the ritual).
On the 21st step, the soldier turns and faces the Tomb for 21 seconds.
The soldier then turns to face the other way across the Tomb and changes his weapon to the outside shoulder.
After 21 seconds, the first step is repeated.
This is repeated until the soldier is relieved of duty at the Changing of the Guard.
The mat is usually replaced twice per year: before Memorial Day and before Veterans Day. This is required because of the wear on the rubber mat by the special shoes worn by Tomb Guards. The sentinels have metal plates built into the soles and inner parts of their shoes to allow for a more rugged sole and to give the signature click of the heel during maneuvers. The sentinels wear sunglasses because of the bright reflection from the marble surrounding the Tomb and the Memorial Amphitheater.
On the ground not covered by the mat, a wear pattern in the tile can be seen that corresponds to the precise steps taken during the changing of the guard. On the mat itself, footprints worn in by standing guard are also visible.
During hurricane Isabel in 2003, the President issued a Presidential Proclamation relieving The Guard during the storm, but The Guard said, "Thank you Sir, but No Thanks!" and stood their post, as they did again during the latest storm. One Guard had to take refuge in the guard shack at the end of the walk for some 75 knot winds, but did not leave his post.