FOR SEVEN YEARS, SOVIET AGENTS SPIED ON THE AMERICAN EMBASSY UNDETECTED  

In 1945, Soviet children presented the ambassador with a gift to commemorate the friendship of the US and the Soviet Union. A hand-carved wooden replica of the Great Seal of the United States.  

The ambassador was so pleased with the carving that he hung it in his office at the official ambassador residence in Moscow: the Spaso house. 

But the wooden carving was more than just a decoration.  

Now hanging on the wall, just behind the ambassador's desk was a state of the art, high-frequency listening device that was completely undetectable.  The intelligence community called the device "The Thing".   

And The Thing was on the office wall of five different ambassadors over 7 years.  

This was an electronic listening device that -- didn't require electricity.  

Let's find out why.