In the summer of 2022, a team of marine biologists bobbed on a small research vessel off the coast of Belize, their attention focused on the sun-drenched waters of the Western Caribbean. They were conducting routine conservation research, tagging tiger sharks along the sheer drop-offs of the Glover’s Reef Atoll. The air was thick with tropical humidity, and the surface water temperature hovered near a warm eighty degrees Fahrenheit. As they hauled in a deep-set longline from the edge of the shelf where the reef plunges into a thousands-of-feet-deep abyss, the line grew unexpectedly heavy. What emerged from the turquoise depths was not the sleek, powerful silhouette of a tropical reef predator. Instead, the scientists confronted a sluggish, massive creature that looked like a living relic from another geological epoch. It was a slow-moving giant with dark, slate-gray skin, a blunt snout, and pale, clouded blue eyes th...