Within this huge national park (over two million acres spread across Wyoming, Montana, and Idaho) are steaming geysers, hot springs, bubbling mudpots, and fumaroles, or steam vents. Drives on the main roads of Yellowstone take tourists through the major hot attractions, which also include Norris Geyser Basin, Upper and Lower Geyser Basin, West Thumb, and Mud Volcano. This unfenced territory, however, steams and smokes off the beaten track as well. That is because Yellowstone "preserves the largest hydrothermal area on the planet," as the official trail guide touts. Its hidden volcanic, tectonic, and hydrothermal forces constantly are at work underground reshaping the landscape.