In the aftermath of the Civil War, the United States expanded the exploration of her western frontiers to gain a measure of the vast lands and natural resources in the region now occupied by our Rocky Mountain States. As part of this effort, the Geological and Geographical Survey of the Territories was organized within the Department of the Interior, and staffed by a group of hardy, pioneering scientists under the leadership of geologist F. V. Hayden. During the summer of 1871, these men, accompanied by photographer William H. Jackson and artist Thomas Moran, made a reconnaissance geological study of the legendary and mysterious 'Yellowstone Wonderland' in remote northwestern Wyoming Territory. The scientific reports and illustrations prepared by Hayden and his colleagues, supplementing the startling accounts that had been published by members of the famous Washburn-Doane Expedition a year earlier, erased all doubts that this unique land was eminently worthy of being set aside 'for the benefit and enjoyment of the people.' By Act of Congress on March 1, 1872, our first National Park was established. During the past century, 50 million people have toured Yellowstone National Park, marveling at its never-ending display of natural wonders. No doubt many have paused to wonder about the origin of these unusual and complex geological features - a question, needless to say, that has intrigued and challenged scientists from the very first days of the Hayden Survey. During the past decade a group of U. S. Geological Survey scientists, in cooperation with the National Park Service and aided by the interest of the National Aeronautics and Space Administration in remote sensing of the geologic phenomena, has been probing the depths and farthest corners of the Park seeking more of the answers. Some of the results of this work, and those of earlier studies, are described in this book to provide a better understanding and enjoyment of this great National Park.