The enigma of long-delayed echoes has perplexed scientists since their initial detection in 1927. (Image credit: Anton Petrus/Getty Images)

In 1927, Norwegian civil engineer and amateur radio operator Jørgen Hals made an unusual discovery: while monitoring transmissions from a shortwave radio station in Eindhoven, The Netherlands, he began hearing odd, "delayed" radio echoes, arriving approximately three seconds after the original transmissions, that sometimes accompanied the station's signals.

"From where this echo comes I cannot say for the present," Hals wrote of his peculiar discovery, adding that he could "only confirm that I really heard it."

Almost a century after Hals' discovery, the exact cause of this unusual phenomenon — known today as long delayed echoes, or LDEs — remains unknown. Their baffling nature has prompted a range of potential explanations, most of which focus on a variety of unexplained natural phenomena occurring in Earth's atmosphere .…