The POW/MIA flag was first recognized on August 10, 1990.

On September 19, 1957, the United States detonated a 1.7-
kiloton nuclear weapon in an underground tunnel at the
Nevada Test Site, a 1,375-square-mile research center
located 65 miles north of Las Vegas. 
 

The test, known as Rainier, was the first fully contained
underground detonation and produced no radioactive
fallout. A modified W-25 warhead weighing 218 pounds
and measuring 25.7 inches in diameter and 17.4 inches
in length was used for the test.

Rainier was part of a series of 29 nuclear weapons and
nuclear weapons safety tests known as Operation
Plumbbob that were conducted at the NTS between
May 28, 1957, and October 7, 1957.