David Helvarg, a longtime environmental activist and journalist, is the founder and president of the marine conservation group Blue Frontier Campaign. He is the author most recently of Forest of the Sea: The Remarkable Life and Imperiled Future of Kelp (Island Press). In a matter of decades, a spectacular cold-water paradise thirty million years in the making is succumbing to warming oceans. Kelp forests are largely out of sight, hidden under the ocean’s surface, yet they are one of Earth’s most wondrous and underappreciated marine habitats.