Alien invaders covet our ocean deeps and start a war with humans to submerge the whole planet. Their weird incursions to the surface are terrifying and genuinely alien. The rising sea levels depicted in the story are similar to today's cli-fi, but were written decades before it became a thing.

This was Wyndham's first book after "The Day of the Triffids", and while it falls short of that masterpiece, it retains some of its vivid inventiveness. The husband and wife who operate as professional equals reflects Wyndham's own relationship, typical of Wyndham's progressive views, so contrary to a lot of the sexist tripe which pervaded the genre in those days.