At the beginning of the 20th century, American astronomer Percival Lowell popularised the idea that long dark lines on Mars were canals built by intellectual Martians.
Signals from Mars
Nikola Tesla received signals that repeated the numbers 1, 2, 3 and 4. He claimed they came from Mars, but research suggests they were radio emanations from Jupiter.
Gennady Sholomitskii believed a powerful variable radio emission represented a signal from a super-civilisation. It was later identified as a quasar, designated CTA 102.
Little green men
Jocelyn Bell and Anthony Hewish discovered a 1.3373-sec signal via radio telescope. They named it ‘little green men’ (LGM-1), but it was actually the first pulsar to be discovered (CP1919).
The Very Large Array (VLA) telescope at Socorro, New Mexico, recorded five highly energetic low-frequency radio emissions (GCRT J1745-3009) in 2002. Some claim this to be an extraterrestrial signal, but it is more likely caused by a pulsar or neutron star.