At this very moment, two spacecraft move at over thirty-four thousand miles per hour, about ten billon miles away from us, each carrying a gold-plated copper record. The spacecraft, Voyager 1 and Voyager 2, are meant as messengers: they carry information about our address in the solar system, the building blocks of our scientific knowledge, and a small sampling of images, music, and greetings from around the world. They also carry whalesong.

      The long squeaks and moans on the record belong to humpback whales. In the 1970s, when the Voyager mission launched, our view of whales was rapidly changing, from game animals to cultural icons and symbols of a nascent environmental movement. Scientists had recently discovered that male humpbacks produce complex songs, composed of phrases collected under broader themes, nested like Russian dolls that repeat in a loop. Humpback whalesong has evolved even since we started listening as each new singer improvises on the loop, creating new structures and hierarchies that constantly change over years, and across ocean basins.

      Whalesong, however, remains a riddle to anyone who isn’t a humpback whale. We can capture its variation, details, and complexity, but we don’t know what any of it truly means. We lack the requisite context to decipher and understand it—or, really, any part of cetacean culture. Even so, we send whalesong into interstellar space because the creatures that sing these songs are superlative beings that fill us with awe, terror, and affection. We have hunted them for thousands of years and scratched them into our mythologies and iconography. Their bones frame the archways of medieval castles. They’re so compelling that we imagine aliens might find them interesting—or perhaps understand their otherworldly, ethereal song.

      In the meantime, whales here on Earth remain mysterious. They live 99 percent of their lives underwater, far away from continuous contact with people and beyond most of our observational tools. We tend to think about them only when we glimpse them from the safety of a boat, or when they wash up along our shores. They also have an evolutionary past that is surprising and incompletely known. For instance, they haven’t always been in the water. They descend from ancestors that lived on land, more than fifty million years ago. Since then, they transformed from four-limbed riverbank dwellers to oceangoing leviathans, in a chronicle we can read only from their fossil record, a puzzle of bone shards unevenly spread across the globe.

      The little we have learned about whales leaves us unsatisfied because the scales of their lives and facts of their bodies are endlessly fascinating. They are the biggest animals on Earth, ever. Some can live more than twice as long as we do. Their migrations take them across entire oceans. Some whales pursue prey with a filter on the roof of their mouth, while others evolved the ability to navigate an abyss with sound. And then they speak to one another with impenetrable languages. All the while, in the short clip of our own history, we’ve moved from heedlessly hunting them to an awareness that they have culture, just as we do, and that our actions, both direct and indirect, put their fate in jeopardy.

      A paleontologist is a good tour guide for what we know about whales, not just because their evolutionary history is profoundly interesting. It’s because we, as paleontologists, are used to asking questions without having all of the facts. Sometimes we’re losing facts: fossils removed from their medium lose clues of context; promising bonebeds are razed to make room for roadways; or bones lay misidentified in a museum drawer. When faced with these challenges, paleontologists turn to inference, drawing on many different lines of evidence to understand processes and causes that we cannot directly see or study—the same approach used by any detective, really. In other words, thinking like a detective is a useful approach to confront the mysteries posed by the past, present, and future of whales.

      This book is not a synoptic, comprehensive account of every different species of whale—there are far too many whales to fit into anything shorter than an encyclopedia. Instead, this book presents a selective account, a kind of travelogue to chasing whales, both living and extinct. I describe my experiences from Antarctica to the deserts in Chile, to the tropical coastlines of Panama, to the waters off Iceland and Alaska, using a wide variety of devices and tools to study whales: suction-cupped tags that cling to their backs; knives to dissect skin and blubber from muscles and nerves; and hammers to scrape and whack away rock that obscures gleaming, fossilized bone.

      The narratives in this book group into three general sections: past, present, and future. Broadly, I want to answer questions about where whales came from, how they live today, and what will happen to them on planet Earth in the age of humans (a new era that some scientists call the Anthropocene). But these stories don’t cleanly fit into these three temporal silos. Instead, they build on one another and reciprocate because the ways that we need to think about whales require thinking about all the evidence at hand: unraveling the many mysteries of living whales requires a background about their evolutionary past, just as much as the surprises from the fossil record can clarify the meaningful facts about their lives today and into the future.

      The first part of the book tells the chronicle of how whales went from walking on land to being entirely aquatic, relying on evidence from the fossil record showing what the earliest whales looked like. These fossils show us details that we couldn’t otherwise know about the history of whales, and I explore exactly how we dig up these clues in the first place. Following fossil whale bones brought me to the Atacama Desert of Chile, where my colleagues and I puzzled over an ecological detective story with the discovery of Cerro Ballena, the world’s richest fossil whale graveyard. How did this site come about, and what does it tell us about whales in geologic time?

      The second part examines how and why whales became the biggest creatures ever in the history of life. The challenges of studying organisms as large as the largest species of whales means thinking about the limits of biology, and what exactly organisms at these superlative scales need to do, on a daily basis, to sustain their enormous sizes. While trying to connect muscle to bone at a whaling station, I share another serendipitous find: the discovery of an entirely new sensory organ in whales. What does an organ, lodged right at the tip of a whale’s chin, mean for how, when, and why baleen whales evolved to become all-time giants?

      Lastly, the third part explores the specter of the uncertain future that we share with whales on Anthropocene Earth. In the twentieth century alone, whaling in the open oceans killed more than three million whales, reducing many populations to shadows of their baseline abundances. Despite this decimation, no single species went extinct until the first decade of the twenty-first century. Since then, not a whistle or splash of the Yangtze river dolphin has been recorded, and responsibility for the extinction of this species can be placed squarely on our shoulders: we dammed the only river in which it lived. Other species, such as the vaquita, remain on the extinction watch list, numbering fewer than one dozen or two dozen individuals. But the news from the field isn’t entirely dire: some whale species have rebounded from the brink, even expanding to new habitats as climate and oceans change. What can we imagine about our shared future with whales, drawing on their lives today and what we know about their evolutionary past?

      Ultimately, the quest to understand whales is a human enterprise. This book is a story not just about knowing whales but also about the scientists who study them. The scientists described in these stories come from a variety of different disciplines, ranging from cell biology and acoustics to stratigraphy and parachute physics. Some are historical but very much knowable through their writings, their specimen collections, and the intellectual questions that they asked. One of the great privileges of my professional