The cover text reads:

Fluctuations in solar radiation have melted the ice caps, sending the planet into a new Triassic Age of unendurable heat. London is a swamp; lush tropical vegetation grows up the walls of the Ritz and primeval reptiles are sighted, swimming through the newly-formed lagoons. Some flee the capital; others remain to pursue reckless schemes, either in the name of science or profit. While the submerged streets of London are drained in search of treasure, Dr Robert Kerans -- part of a group of intrepid scientists -- comes to accept this submarine city and finds himself strangely resistant to the idea of saving it.

First published in 1962, Ballard's mesmerising and ferociously imaginative novel gained him widespread critical acclaim and established his reputation as one of Britain's finest writers of science fiction.

This is #563 in the https://www.listchallenges.com/1001-books-you-must-read-2018 list

Well this is an interesting concept in that the cause of the breakdown of society is not nuclear war, or lab-released plague, or over-population or anything man made, but purely a natural phenomenon. The world is warming, sea levels are rising, and most of the remaining humans are retreating as far from the equator as they can get. But there are some for whom a primordial urge sets them on a different path.

relocated from Kapper Eric's reading table and
left in Marleen's minibieb

And today, by chance, I met Marleen :-)
but I didn't explain BookCrossing well :-(