A pioneering marine biologist takes us down into the deep ocean to understand bioluminescence, the language of light that helps life communicate in the darkness, and what it tells us about the future of life on Earth. Edith Widder grew up determined to become a marine biologist. But after complications from a surgery during college caused her to go temporarily blind, she became fascinated by light as well as the power of optimism. Her focus turned to oceanic bioluminescence, a scientific frontier in our last earthly one, and with little promise of funding or employment she took a leap into the darkness. On her first visit to the deep ocean, in an experimental diving suit that took her to a depth of eight hundred feet, she turned off the suit's lights and witnessed breathtaking explosions of bioluminescent activity. Concerns about her career went out the window. She just wanted to know one thing: Why was there so much light down there? Below the Edge of Darkness takes readers deep into our planet's oceans as Widder pursues her questions about one of the most important and widely used forms of communication in nature. In the process, she reveals hidden worlds and a dazzling menagerie of behaviors and animals, from microbes to leviathans, many never-before-seen or, like the legendary Giant Squid, never-before-filmed in its deep-sea lair. Alongside Widder, we experience life-and-death equipment malfunctions and witness breakthroughs in technology and understanding, all of it set against a growing awareness of the deteriorating health of our largest and least understood ecosystem. This book is an adventure story as well as a science story. But it's also about the sometimes complicated business of exploration. And ultimately, Widder shows us that exploration, and the corresponding senses of discovery and wonder, are the keys to the ocean's salvation and thus our future on this planet.