It was Brian Aldiss that accused John Wyndham of writing "cosy catastrophes" but there is nothing cosy about the catastrophe depicted here.
Some form of alien beings arrive from space and settle in our deepest oceans and, even though they cannot exist in the low pressure environment of the surface and we can't exist in their high pressure environment at the bottom of the oceans, it soon becomes clear that the two cannot cohabit the earth and that one of us must go.
I say it becomes clear but as far as humanity is concerned, it takes impossibly long for the penny to drop for all but a few fringe "scare mongerers". In this story, the public seem to be extraordinarily resistant to coming to terms with the true nature of the threat, the full extent of their predicament and the need for urgent action. Not that there seems to be anything that can
be done, humanity is on the back foot forced to be strictly reactive to a threat who's precise nature remained a mystery throughout the book.
The tone of the book is depressingly doom laden. The governments, helpless in the face of this unknown threat seem capable of doing no more than soothing the worries of their public and putting a brave face on things as the humanity's domain is encroached upon ever further.
We follow the story through the eyes of husband and wife journalist team as they observe events usually from a distance, but sometimes at the forefront as they unfold and civilization is gradually brought to its knees and begins to unravel. The emphasis for much of this book is on the media reaction the way public perception shifts accordingly.
Personally, I thought the narrative style was somewhat distancing for large parts of the book and it may have benefited from multiple points of view to keep the reader close to the events that were taking place but some of the scenes were very evocative, depicting quite horrific moments when the protagonists happened to be close to the action.
Not my favourite Wyndham novel but certainly has a lot going for it nonetheless.