Long-delayed echoes
In 1928 a short pulse sent from Eindhoven came back to Oslo as an echo seconds late, timed by the physicist Carl Størmer.
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The Eindhoven Echoes and the Seconds Nobody Could Place
Long delayed echoes are radio signals that return seconds after they were sent, far later than any path around the Earth allows. They were first reported in 1927 by Jorgen Hals in Norway, from the Dutch station PCJJ, and timed in 1928 by Carl Stormer and Balthasar van der Pol. In 1973 Duncan Lunan read them as a message from a probe. Here are the records, the readings and what is still unexplained.