For decades, people around the world have reported hearing an unexplained low-frequency. The elusive phenomenon has inspired theories involving everything from ocean waves to UFOs. One study, discussed in a new Pop Mech story called “A Mysterious Hum Has Plagued the Earth for 50 Years. Scientists May Have Finally Found Its Source” suggests the answer may lie in the human ear. Watch the full episode above now, and check out more installments of “The Astounding Pop Mech Show.”