This is a crucial book for those fascinated by the Ufo mystery, or Cryptozoology. It is also a classic of Fortean literature. Sanderson who died in 1973 was a naval intelligence officer during WW2 and coined the term "Cryptozoology".
The observational premise and the starting point of this book is that by far and away the majority of Ufos are seen near bodies of water, often entering and leaving the water. Sanderson collected hundreds of case studies some of which are tabulated at the back of the book. He emphasises that the hydrosphere covers most of the Earth and that this is, quite uniquely, a "water planet". Sanderson calculates the enormous volume of the hydrosphere where unknown craft may be lurking, compared to the tiny amount of inhabited land. He also points out how narrow are the shipping lanes and air corridors, where sightings originate. There are studies of cases where ships and aircraft have disappeared over the sea, and others where only the humans on board have inexplicably disappeared. Sanderson has collected a good deal of data concerning vortices, and the strange phenomena that they can produce, perhaps the OINTS (Other Intelligences) have mastered their power.
The book is full of open- minded speculation, which is the basis for truly scientific hypotheses; he collects all of the data however bizarre it might seem. Perhaps Sanderson does not reveal everything that he knows. He does not rule out the possibility that some of the Ufos, Usos or Uaos as he calls them are produced by humans, he "speculates" that there may be underwater bases hollowed out of undersea mountains and continental masses. Nothing can be ruled out.
By the end of the book however, the reader is convinced that Sanderson really does believe in non- human technologically superior OINTS which have created a civilisation under the seas and oceans. He also possesses case studies of Uaos diving into rivers and lakes. This aquatic civilisation or civilisations could have evolved throughout the geological ages or it may have come here from elsewhere in the universe.