Books and articles about UFOs now number thousands.

In past centuries people lacked concepts of "solar system", "earthlike planet", "galaxy", "spaceship", "evolution" and "extraterrestrial". Consequently no one reported seeing any Martians in flying saucers.

Last century ideas in evolution and astronomy became common knowledge. This got some people speculating about life on other planets. H.G. Wells popularized this idea in his War of The Worlds (1898), a story of Martians invading planet Earth.

In 1938 a radio dramatization of War of The Worlds was taken seriously by the public and sparked panic. Hundreds rushed to fight the invaders. Many people reported sightings of, or encounters with, strange monsters.

The UFO phenomenon began in earnest in 1947. Kenneth Arnold, a businessman, was flying his plane in Washington State when he saw nine pie-shaped metallic objects going at almost 2,000 kilometres per hour. The media played the story up and many people became convinced. US Air Force investigators concluded Arnold had hallucinated. Nevertheless, flying saucer reports soon came in by hundreds.

Project Blue Book, official Air Force investigations, lasted 1952 to 1969.

Hollywood soon cashed in. So did cranks (or shrewd exploiters?) who wrote books about their trips in UFOs. Stimulated by Cold War tension the Air Force investigated everything, no matter how far fetched, but found nothing substantial.

In 1972 J.H. Hynek, an astrophysicist accounted for the lack of hard evidence with a conspiracy theory of official UFO cover up. Some UFO buffs, believing that other Solar Systems were too distant to be the source of saucers placed their origin inside the hollow Earth.

In 1978 official U.S. interest in UFOs resumed.