In 1963, Friedrich Jürgenson, a Swedish bird-watcher, purchased one of the early portable tape recorders to sample bird song in remote areas. In 1959, while monitoring one of his recordings, he noticed strangely garbled fragments of seemingly human speech had somehow made their way onto the tape, although he was absolutely sure that he had been completely alone when he made the recording. He was so mystified that he started experimenting, successfully recording a huge number of similarly extraneous voices during the next four years.

While the voices were mostly distorted ­ resembling those of the Daleks from Dr Who ­ they seemed to belong to intelligent beings. Most were barely intelligible and almost completely drowned by tape hiss, which was a serious problem with the open reel recorders of the time.

By closely listening to the voices, Jürgenson found that they spoke in different languages, often changing to another idiom in mid-sentence. Also, longer phrases often had an improper structure or grammar and, in some cases, the syllables were either stretched or compressed in a way that made it quite hard to comprehend the messages.

The strangest aspect of all was the uncanny way the voices seemed to respond to his comments and even to mention his name now and then. Jürgenson began to hold conversations with the strange voices by recording questions and later searching the tape for answers. When he published his findings in 1963, Jürgenson was completely convinced that he knew the origin of the 'tape voices' ­ in his opinion, the tape recorder was acting as a form of electronic communication link to the realm of the dead...