What is it about eels? Depending on who you ask, they are a pest, a fascination, a threat, a pot of gold. What they are not is predictable. Eels emerged some 200 million years ago, weathered mass extinctions and continental shifts, and were once among the world’s most abundant freshwater fish. But since the 1970s, their numbers have plummeted. Because eels—as unagi—are another thing: delicious.

In Slippery Beast, journalist Ellen Ruppel Shell travels in the world of “eel people,” pursuing a burgeoning fascination with this mysterious and highly coveted creature. Despite centuries of study by celebrated thinkers from Aristotle to Leeuwenhoek to a young Sigmund Freud, much about eels remains unknown, including exactly how eels beget other eels. Eels cannot be bred reliably in captivity, and as a result, infant eels are unbelievably valuable. A pound of the tiny, translucent, bug-eyed “elvers” caught in the cold fresh waters of Maine can command $3,000 or more on the black market. Illegal trade in eels is an international scandal measured in billions of dollars every year. In Maine, federal investigators have risked their lives to bust poaching rings, including the notorious half-decade-long “Operation Broken Glass.”

Ruppel Shell follows the elusive eel from Maine to the Sargasso Sea and back, stalking riversides, fishing holes, laboratories, restaurants, courtrooms, and America’s first commercial eel “family farm,” which just might upend the international market and save a state. This is an enthralling, globe-spanning look at an animal that you may never come to love, but which will never fail to astonish you, a miraculous creature that tells more about us than we can ever know about it.