Immanuel Velikovsky (1895-1979) was by profession a doctor of medicine, specialising in psychiatry. However, his fame is based on being arguably the most controversial catastrophist of the 20th century. He daringly proposed that the Earth had a number of close encounters with other planetary bodies that resulted in catastrophic consequences, including interference with the rotation of our planet. He speculated that Atlantis was probably destroyed during one of these cataclysmic events.
Some have seen the influence of Ignatius Donnelly’s Ragnarok, written seventy years earlier, in Velikovsky’s cosmic collision theories. In fact a number of commentators have noted how Velikovsky seemed reluctant to credit earlier writers, such as W. C. Beaumont and Johann Radlof (1775-1846)(b), with their contributions to the development of the theory of planetary catastrophism. Rens Van Der Sluijs has written an interesting two-part paper(d)(e) listing the catastrophists who preceded Velikovsky demonstrating a certain lack of originality on his part! Others take a more critical view of his ideas(g). In 1950, he responded to this criticism with a defensive piece(n), but I consider it inadequate as he continued to ignore the work of Radlof and Beaumont.
Van Der Sluijs has written a two-part(k)(l) article on Velikovsky’s radical views regarding Venus as a comet-like body and how Aztec sources support some of his contentions.
Carl Sagan (1934-1996), was a well-known American astronomer, author and lecturer. He is considered a leading debunker of Velikovsky’s theories. He devoted much of his Broca’s Brain  to this end. Charles Ginenthal (1934-2017) produced an extensive rebuttal of Sagan’s criticisms in Carl Sagan & Immanuel Velikovsky .
Velikovsky was initially inclined to link the disappearance of Atlantis with the eruption of Thera, but later came to support a location between the Azores and the Mid-Atlantic Ridge(i). He was an early questioner of Plato’s figure of 9,000 years for the age of Atlantis, suggesting that it was exaggerated by a factor of ten[0037.152]. ”Whatever the source of the error, the most probable date of the sinking of Atlantis would be in the middle of the second millennium, 900 years before Solon, when the earth twice suffered great catastrophes as a result of ‘the shifting of the heavenly bodies.’ These words of Plato received the least attention, though they deserved the greatest.”
Velikovsky offered intriguing evidence that on least one occasion the early Egyptians experienced the sun rising in the west and setting in the east(q)! His other major contribution was in his questioning of the accepted Bronze Age chronologies of the eastern Mediterranean. Later writers, such as David Rohl and Peter James have built on his chronology work, while Gary Gilligan has added support for Velikovsky’s planetary theories as well.
One website(a) provides us with a considerable amount of Velikovsky’s unpublished work, while another offers an encyclopedia of his work(c). A more general German site(f), in English, is also worth a visit.
The three of Velikovsky’s most popular books as well as some of his lesser known papers are available as pdf files(j)(m).
Jan Sammer was an assistant to Velikovsky (1976-1978) and an archivist and editor for the Velikovsky Estate (1980-1983). He advises us that he was involved in the completion of Velikovsky’s unpublished book, In the Beginning, which can be read online(h).
In 2012, Laird Scranton, published The Velikovsky Heresies, in which he reviews Velikovsky’s controversial theories in the light of scientific discoveries since his death. Not unexpectedly, Scranton does find evidence that supports some of Velikovsky’s contentions.
Some readers may wish to see a video by Wallace Thornhill, of Electric Universe fame, in which he discusses Velikovsky’s Astrophysics (o). There are a number of free papers and books, including some Velivovskian material, available online(p).