Early maps marked uncharted seas as dragon territory; later explorers treated ocean depths as lifeless wastelands. As Casey shows in her vivid account of human forays into the deep, the first estimations were closer to the truth—the colorful monsters detailed on Olaus Magnus’s 16^th century Cartus Marina, for instance, while imaginary, heralded the actual bioluminescent creatures, bat-like sponges, “crystal-white anemones,” and startling snail fish to come when technology at last gave us access to the deepest, abyssal, or hadal (yes, from hades) layer of ocean with its absolute darkness and crushing weight of “eleven hundred atmospheres.” Casey makes the machines—Beebe’s bathysphere, OceanX robotic subs--as much part of her adventure as the trenches, ocean volcanoes, hydrothermal vents, and “undulating carpet of fine gold sediment” they reveal (along with the mountains of plastic, dumped munitions, drift nets, and other human waste).  Her awe-inspiring book is at once rigorous science and a “psilocybin vision.”