Beneath the surface of the world's rivers and lakes swim mysterious giants, the real-life Loch Ness monsters and Bigfoots of the aquatic world. They are a diverse assemblage of poorly understood creatures, from gargantuan gars to sumo-sized stingrays. These ancient fish-some who have been around for hundreds of millions of years-play critical roles in their freshwater ecologies. Threatened by overfishing, habitat loss, dams, pollution and climate change, the majority of the world's freshwater megafish are today at risk of extinction. As an aquatic ecologist and host of the National Geographic Channel's "Monster Fish" series, Zeb Hogan has spent more than a decade searching for and studying these increasingly endangered river titans. In this book, he teams up with award-winning journalist Stefan Lovgren to tell for the first time the remarkable and troubling stories of the world's megafish and chronicles the race against the clock to find and protect these ancient leviathans before they disappear forever.