According to Life magazine of Sept. 8, 1952(g), after twenty years of research, he organised a dive around Heligoland and found extensive underwater structures. These explorations led to his book Das Entraselte Atlantis  being published in 1953. It was later translated into English and published as Atlantis of the North  and is now available to read on the internet(a).
Spanuth’s basic thesis was that following a major catastrophe in the North Sea around 1250 BC, the Mediterranean experienced an invasion of Scandinavians, whom he referred to as the ‘North Sea Peoples’. Part of the physical evidence he produced was the horn-helmeted Sea Peoples depicted at Medinet Habu. Since we are all used to seeing Vikings depicted with horned helmets, many are surprised to find that it is a late 19th-century invention(e)(f).
Spanuth’s theory implies that such helmets had been a standard army issue in the region for over a millennium. In fact, the Vikings used rather plain helmets which they did not manufacture themselves but traded for them from other Germanic peoples in mainland Europe(d). On the other hand, one of the Sea Peoples, the Shardana, generally believed to have come from Sardinia, did use horned-helmets. However, there are aspects of this claim that are the subject of continuing debate, but the suggestion of a North Sea connection has weakened considerably.
Spanuth considered Basileia, the royal island of Atlantis, to have been located near Heligoland. He produced a mass of evidence to support his views but found his book under severe attack by many academics, which, in general, had the support of the public. After being publicly labelled, among other things, a liar, Spanuth was forced to challenge his detractors in the courts. After some six years, he was vindicated when ten professors withdrew their plea, admitting that their arguments against the pastor were untenable. Felix R. Paturi has more information[1339.215] on this disgraceful episode, as well as a note of scientists who supported Spanuth.
>The late Bernhard Beier expanded on the list of academics who did endorse Spanuth(i)(j) and in some ways more interesting is an article he wrote giving an account of the efforts of many academics to silence Spanuth completely. Coincidentally, around the same time, Immanuel Velikovsky was under a similar attack in the USA.<
A study of Spanuth’s references would suggest that he had access to the prehistoric research archives of the Ahnenerbe and has successfully collated and analysed a lot of this extensive material in his books. Vidal-Naquet bluntly labels him a Nazi[580.124], although his publisher, Wolfram Zeller, denied it. It may be relevant to mention that in the 1930s, Heinrich Pudor an avowed German anti-Semite also proposed Helgoland as Atlantis, but I have been unable to find any reference to Pudor by Spanuth!
The German Wikipedia claims that Spanuth was a member of the NSDAP (Nazi Party) from 1933 until 1945. Similar claims that he had been in the SS have also been refuted(c). In 2002, Frank Doenenburg, on his website(b) discussed Spanuth’s politics at length. In my view, all these matters, however unsavoury, risk distracting us today from discussing dispassionately the merits or otherwise of Spanuth’s Atlantis theories.
Spanuth’s second book had a much better reception. His final offering was Die Atlanter(1976), which was also published in English, however, this is just a revised and expanded version of his 1965 book.
Atlantisforschung has highlighted Spanuth’s opposition to the Minoan Hypothesis in a discussion paper on its website. I have published here a translation of a short excerpt from Die Atlanter that shows his disdain for the idea of an Aegean Atlantis.
“Neither Thera nor Crete lay in the ‘Atlantic Sea’, but in the Aegean Sea, which is expressly mentioned in Crit. 111a and contrasted with the Atlantic Sea. Neither of the islands lay at the mouth of great rivers, nor did they “sink into the sea and disappear from sight.” ( Tim. 25d) The Aegean Sea never became “impassable and unsearchable because of the very shallow mud”. Neither Solon nor Plato could have said of the Aegean Sea that it was ‘still impassable and unsearchable’
or that ‘even today … an impenetrable and muddy shoal’ ‘blocks the way to the opposite sea’ ( Crit. 108e). Both had often sailed the Aegean Sea, and their contemporaries would have laughed at them for telling such follies.”(h)
Spanuth has still a lot of supporters and is constantly referred to, particularly by German investigators such as Arn Strohmeyer and Gerhard Herm. Felice Vinci, who strongly favours a Northern European origin for Homer’s epic tales, also places Atlantis in a northern context. The Danish writer, Kirsten Bang, published a short book  in which she also placed Atlantis in the Wadden Sea where Helgoland is located. She also supports a date of 1300 BC for its destruction.
Another recent supporter of Spanuth’s Atlantis theory is Holger Kalweit who has written a trilogy, the first of which is Irrstern über Atlantis . This initial volume is concerned with the destruction of Atlantis by a comet (Phaeton) in 1222 BC, leaving Helgoland as a remnant. Refugees fled south to the Eastern Mediterranean leaving their cultural imprint on the region. Unfortunately. this huge 700-page book is to be followed by two more in which the author moves on to expand on the subject of ‘lizard people’, which for me has him as a fully paid-up member of the lunatic fringe.
(b) https://www.fdoernenburg.de/Forum/viewtopic.php?f=28&t=1213 (page closed, July 2017)