This is a strange sort of ark: a brick warehouse in Knoxville, Tennessee. Not only will the thing never float, but the life-changing flood is all inside, where water pours day and night from a maze of pipes into 600 glass aquariums and plastic tubs stacked to the ceiling. The passengers, most just a few inches long, are fish: madtoms and darters, topminnows and chub. For them the carefully filtered, aerated water offers the breath of life, whereas their natural homes—streams and rivers in the southeastern United States—are choked by dams and clouded with pollutants. The fish aboard the ark are among the last of their kind.