Along with the 76th anniversary of Pearl Harbor, this week also marks the 73rd anniversary of another infamous event. December 6, 1944, a Japanese Fu-Go balloon bomb blew up just outside of Thermopolis, Wyoming.

At around 6:15 p.m., four workers at a coal mine heard a whistling noise overhead, followed by an explosion that sent flames parachuting across the night sky.

The bomb was one of over 9,300 hydrogen fire balloons launched from Japan and the first to land in the United States. Although it was the first intercontinental weapon, the campaign was a failure. While several hundred balloons eventually made landfall, they did very little damage and were responsible for only six casualties.

The Japanese Navy had hoped the offensive would encite a panic. Instead, the United States War Department issued a censorship order, prohibiting newspapers from reporting the origin of the mysterious balloons.

While the bomb in Thermopolis was the only documented explosion in Wyoming, other balloons may have landed in the state. Over the years, hundreds of fragments from Japanese balloon bombs have been discovered in remote areas of Alaska, California, Oregon and Washington.