In the nineteenth century, the seas went dark each night. Ships were isolated by distance and silence, and a single mistake could mean disaster. From that darkness emerged a woman whose determination would change maritime history. Widowed at just twenty-one and left to support four young children, Martha Coston uncovered her husband's unfinished sketches for a naval signaling system. With no formal scientific training and little support, she taught herself chemistry through dangerous experimentation, risking her health to create something the world desperately needed; reliable light in the darkest hours at sea. Her invention transformed naval communication, allowing ships to speak across black waters using coded bursts of color. During the Civil War, millions of her signal flares guided fleets, coordinated maneuvers, and saved lives. Yet even as her work became indispensable, Martha fought tirelessly for recognition and fair compensation in a system determined to overlook her. The Woman Who Lit the Seas tells the remarkable true story of an inventor history nearly forgot. It is a powerful biography of resilience, ingenuity, and quiet courage; and a testament to the light one woman refused to let be extinguished.