Deep dark sea people.
The rifters are people genetically modified to work in the very deep ocean. Some of them are certainly not there by choice, being given the jail/other nasty fate option and taking this one.
They are needed because the biosphere is fragile and in big trouble, and other power sources are needed to keep it going.
So, lots of intrigue, manipulation, fighting, and scientific speculation thanks to Watts' background and lots of research.
He kindly details some sources for some of the ideas at the end.
Call this book a 3.75.
3.5 out of 5