The drowned world.

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.

(via 2014 Fourth Estate edition)