The Crystal City

Polly has always felt different. She is always top in swimming competitions, she swims in the sea but never feels the cold, and has a birthmark shaped like a small fish. Then she meets a girl called Lisa who has the same birth mark. The girls discover that they were adopted after having been found on a beach and that they can swim underwater for long periods...This leads them to a city underwater, a completely different way of life and some startling revelations...