In Berlin, the revelations that the NSA may have listened to Chancellor Merkel's cell phone have created a wave of protest. During the Cold War, American allies established an elaborate espionage system called Echelon to eavesdrop on communication activities in the Eastern Bloc. Back then the German government was aware of US-spying tactics from places like the Teufelsberg in Berlin.

Teufelsberg, or "Devil's Mountain," offered Americans an ideal vantage point over the divided city of Berlin. After World War II some 25 million tons of war rubble were heaped up in a forest on the edge of Berlin to form the city's largest hill at 120 meters. Underneath the debris lies a never completed Nazi military technical college. Today, all that's left of the spy station is a ruin.

After dumping the debris of some 15,000 war-damaged buildings onto the site, the Berlin government covered the hill in vegetation and turned it into a winter sports paradise. Shortly afterwards, the US military discovered the hill provided a perfect elevation point for monitoring flight paths to West Germany and radio and telephone networks in East Germany.

Part of Teufelsberg was turned into a military zone in October 1964 and the construction of the large listening station began. The project was only referred to as "The Hill" by American soldiers. Several secluded buildings as well as five powerful antenna domes were built. It is estimated that more than 1,000 people worked on the project at its busiest time.

The spy station was part of the global Echelon intelligence gathering network created to monitor the military and diplomatic communications of the Soviet Union and its Eastern Bloc allies during the Cold War. It was located in the British Sector, but the Brits and Americans didn't trust each other. So they split up the area in two, built everything twice and double-eavesdropped on their enemies.

Almost the entire Eastern Bloc was under surveillance, from the governing party in Eastern Germany to Soviet military facilities. Conversations in German, Czech, Polish and Russian were meticulously recorded, transcribed and translated. In a three-stage evaluation system only the most important information was forwarded. The surveillance reached up to 700 kilometers to the east.

After the fall of the Berlin Wall, the listening station was abandoned. In 1992, the Americans took their electronic devices and left the radar domes behind as empty shells. The site was used for civil air surveillance for a short time before being bought by private investors. Their plan was to build apartments and hotels on the hill, but nothing resulted from their preparations.

The abandoned spy station soon became a popular target for vandals. Now windows are broken, old computers smashed, the interior is in ruins and even old pipes were stolen to be sold as scrap. With rusty fences and weathered iron gates, it's clearly stated that visitors enter at their own risk. Holes in the ground, missing railings and unsecured stairwells do not make exploring the area any safer.

What was once a major building site for Nazis and then a surveillance center during the Cold War is now a popular spot for enjoying a view over the German capital and surroundings. Since 2011, the site has been open to the public. The Teufelsberg Community of Interest organizes an open-air electro festival on the premises. The only question left is whether the US is still watching.