He had seen the sentinels stand before the Tomb of the Unknown Soldier nearly a decade ago, watching from a marble colonnade on his eighth-grade field trip.

Sgt. Craig Hudson was with his class from Massachusetts, a witness to an enduring oath to guard and protect the soldiers he would never know.

On Monday, Hudson served four shifts guarding the Tomb, a 24-year-old member of one of the military’s elite regiments.

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During a day of deluge in the District, Hudson paused in the middle of his 26-hour shift, drying off in the underground barracks below the changing of the guard.

In an interview as the storm system rolled through the region, Hudson said nothing changes when precipitation pummels the plaza – a timeless tableau that has remained the same for generations.

“It never becomes more difficult, even in the rain, there’s no difference,” Hudson said. “I just think of what they could have possibly done, who they were.”

Hudson is part of the 3rd U.S. Infantry Regiment, better known as the Old Guard. It is the oldest active-duty infantry unit in the Army, serving since 1784. Hudson trained for seven months to protect the most solemn site in Arlington, on this day, counting 21 paces through relentless rain.

“When the guard is walking the mat, he's counting his 21 seconds, 21 steps,” Hudson said. “And that 21 is significant to military service members, because of the 21-gun-salute during a military funeral.”

The ritual in the rain continues largely in silence – save the commands delivered every half-hour when the guard changes. Members of the 3rd Infantry have watched over the Tomb since 1948, when the unit was reactivated in Washington after World War II.

“When you walk in the rain, you think not just about the task at hand, but about their identity,” Hudson said. “They don't have a name. They gave their name up for this country.”

The Tomb of the Unknown Soldier remains open to the public from 8 a.m. to 7 p.m. April – September and 8 a.m. to 5p.m. October – March. Guards watch over the Tomb 24 hours a day, also serving as the Army’s official ceremonial unit and escort to the president.