Heinerth, a diver, photographer, and filmmaker, is one of the few women in the rarefied field of underwater cave exploration, a “sport” that claims more deaths than mountain climbing. Feeling at home in water as she seldom does on land, Heinerth finds an exhilarating freedom when plunged miles below the surface—despite being strapped into equipment weighing as much as 250+ pounds. In her astounding memoir she recounts her record-breaking dives to map cave networks around the world, including, most spectacularly—and dangerously—her exploration of “the largest moving object on earth,” the Antarctic’s B-15 iceberg. But Heinerth’s rich descriptions of otherworldly seascapes, the fascinating glimpses of the technology that makes long, deep dives possible, her tales of the adventures involved in just getting to the dive sites, and the chilling reports of illness, accidents, and fatalities, are only half her story. The other half is the self-scrutiny of a talented, ambitious woman struggling with doubt, insecurity, and rampant sexism even as she sets records, leads dangerous projects, and proves herself over and over again. Launched into diving as a way to manage two traumatizing experiences when she was in college, she found her calling when she “learned to embrace fear as a positive catalyst in my life.”