As humans, we love superlatives. We want to taste the spiciest pepper and climb the tallest mountain. We’re fascinated by the extreme, from the smallest book to the heaviest hamburger. In Chasing Giants: In Search of the World’s Largest Freshwater Fish authors Zeb Hogan and Stefan Lovgren follow this impulse to unexpected depths.

Hogan is a research biologist at the University of Nevada, Reno, and the host of National Geographic’s TV show Monster Fish. Lovgren is a journalist and filmmaker and regular contributor to National Geographic. While they work together, the book is written from Hogan’s point of view as he traverses the world in search of the largest freshwater fish: Amazonian arapaima, Australian sawfish, Cambodian stingrays. And a whole lot of catfish. The book blends Hogan’s hunt for these underappreciated creatures with revelations from his studies of the Colorado River and his NatGeo show.

It all starts when Hogan learns of a Thai fisherman who caught a 646-pound Mekong giant catfish, “the largest freshwater fish ever recorded,” the authors note. Hogan goes in search of the largest freshwater fish (by weight), but he soon learns that, given the lack of research and interest in freshwater fish and ecosystems, this will prove challenging. “For every peer-reviewed article that focused on freshwater, there were 20 studies on marine issues,” the authors write. It’s hard to find animals that you don’t know exist, let alone protect them, and the world now has a greater number of endangered freshwater ecosystems than marine ecosystems.

Throughout the book, the authors blend Hogan’s quest with similar insights. So while the search reels us in, the discussion of these overlooked animals is the real prize. For instance, the Texas alligator gar, which can be over 300 pounds and more than 10 feet long, has long been called a “trash fish,” frustrating fishermen by destroying their nets and eating the same fish they hoped to catch. But when researchers began more closely studying the gar in the 1990s, they learned the species was critical for the ecology, generally eating fish that were in abundance. Armed with this knowledge, government officials and conservationists can now come up with a science-based plan to protect the species.

Hogan’s travels take him to Australia’s Fitzroy River to seek out the largetooth sawfish, which can grow as long as 21 feet and is best known for its saw-like nose, or rostrum, which has electromagnetic pores to help it hunt. That rostrum also makes the fish, which is prized as a catch for its impressive shape and length, extremely likely to get caught in fishing nets. “The sawfish had evolved for survival,” the authors write. “Now it seems to have evolved for extinction.”

One of the more powerful sections of the book is the discussion of the Colorado River. Hogan and Lovgren write: “Few rivers on Earth have both changed the world through which they flow and had as much change brought upon them as the Colorado River.” This once-mighty river flowed for millions of years before Indigenous peoples fished its waters and hunted its banks. In the nineteenth century, the river was dubbed “the Nile of America” and played a pivotal role in the development and exploitation of the Western US. The United States government built monumental dams up and down the river, and by 1963, the authors explain, “the mighty river that had once carved out America’s most iconic landscape would no longer reach the sea.” The book makes a convincing case for the ecological disaster of dams in the US and all around the world.

With so much insight to offer, the book might have benefited from a cleaner structure, as the authors jump back and forth through time and across geography. (One chapter per fish might have brought more clarity to each species and its natural habitat.) Meandering aside, it makes a strong case for why these freshwater giants need to be studied and protected.

Meanwhile, though, time is running out. During Hogan’s search, several contenders may have disappeared. The freshwater stingray, in Thailand’s Mae Klong River, for example, may have been wiped out by a chemical spill. There’s no question that humans are the largest threat to these incredible creatures, but Chasing Giants reminds us that we can reverse course: by removing dams, conducting more research, and dedicating more resources to these freshwater mysteries.