The World War l Unknown is below the marble coffin. Other Unknowns are beneath the white slabs on the ground to the left of the guard.  

The remains of the Unknown Soldier are committed to the ground during
the burial services of 1921.

The Tomb of the Unknowns (or Tomb of the Unknown Soldier as it is often called) is  
a monument dedicated to U.S. Service members who have died without their remains being identified. It is located in Arlington National Cemetery in Virginia. On November
11, 1921, President Warren G. Harding officiated the interment ceremonies at the Cemetery. The Tomb of the Unknowns has been continuously guarded, 24 hours
a day, 7 days a week, since July 2, 1937.