The Tomb of the Unknown Soldier Sentinels
The Sentinel walks a 63-foot mat in exactly 21 steps, faces the Tomb for 21 seconds, and does it again — around the clock, in any weather, and has since 1937.
How it actually works
Soldiers of the 3rd U.S. Infantry Regiment ("The Old Guard") have guarded the Tomb of the Unknown Soldier at Arlington National Cemetery continuously since July 2, 1937. The walk is exact: 21 steps down the mat, a 21-second pause facing the Tomb, a sharp turn, another 21-second pause, then 21 steps back — the number echoing the honors of a 21-gun salute. The Sentinel's weapon stays on the shoulder away from the Tomb, symbolizing that the Sentinel stands between the Tomb and any threat. The post is guarded every minute of every day regardless of weather; guards have refused direct orders to take shelter during hurricanes rather than leave the Tomb unwatched. Fewer than 20% of volunteers who apply are even accepted into training, and most of those wash out before becoming a certified Sentinel.
A lot of visitors assume the crypt still holds four sets of remains — one Unknown each from World War I, World War II, Korea, and Vietnam. That was true once, but not anymore. In 1998, DNA testing identified the Vietnam Unknown as Air Force 1st Lt. Michael Blassie, and his family had him disinterred and reburied near his home in St. Louis. The Vietnam crypt at Arlington is still there and still guarded — it was rededicated to all missing service members from that era instead of holding a body. The ceremony you see is exactly as real and exactly as demanding as advertised; there just isn't a Vietnam Unknown inside anymore.
Congress approved a WWI Unknown burial at Arlington in 1921; round-the-clock guard began 1937; the Old Guard has held the post since 1948.