In the early twentieth century, a mysterious ecological crisis nearly wiped out the fish that most people cared about in the largest freshwater ecosystem on earth—the Great Lakes. The impact reverberated across the region, ruining local industries, damaging small town economies and indigenous communities, and destroying the livelihoods of people in the United States and Canada. With little reason for hope, a dedicated group of scientists, policymakers, and conservationists tackled the mystery.