Now a somewhat secretive Eritrean military base, Kagnew Station was one of America’s key listening posts for much of the Cold War. Radios located here are able to pick up clear signals from thousands of miles away; local AM stations in Asmara have reported hearing Finnish broadcasters on occasion.
Altitude plays a big role in Kagnew’s magic, as does the relative quiet of the plains below. But those factors alone can’t quite explain why a short-wave radio at Kagnew can pick up Bulgarian chatter with little difficulty. In I Didn’t Do It For You, a history of modern Eritrea, Michaela Wrong offers a scientific guess:
Some experts have speculated that Asmara benefits from another intriguing natural phenomenon—”ducting”—in which radio signals rise through the earth’s atmosphere, bounce horizontally along under the troposphere and return to earth via “ducts” thousands of miles away from their original source.