Phantom Ranch, midway through the Grand Canyon and accessible only by trail, is the sole permanent settlement within the canyon. From there west, the canyon narrows and darkens as the Colorado River relentlessly carves its way through rock that is the oldest in the Southwest. From the north and south sides, canyons so narrow they are known as "slots" reach the churning river. Finally, the canyon the world's greatest geological wonder ends and the tamed river meets the placid waters beyond. Water experts give their take on the river and what it and its waters mean.