Under a Yellowstone landscape populated by interesting animals lies a sleeping giant, a great volcano.This book makes a clever connection between the geology and the wildlife of America's first national park. A gentle text and scenic illustrations depict various animals' activities throughout the day. Then, attention turns to what's happening underground, the processes that result in the bubbling mud pots, hissing steam vents, and roaring geysers. Fox describes a time when the volcano erupted, noting the lasting effects in the rocks and plant cover, and, finally, the narrative returns to the animals, now ready for sleep. The metaphor is accessible even to young listeners, and the effect is soothing rather than frightening. The text and translucent frames for the animal scenes are set directly on double-page illustrations done with pencil, watercolor collage, and digital oil paints. These realistic images show the large animals described in the text—wolf, moose, bear, elk, bighorn sheep, mountain goat, mountain lion, coyote—and smaller ones that can be found by the curious child and easily identified by a knowledgeable adult reader. A helpful map on the title page shows the outlines of the caldera within the park; the backmatter includes a glossary of volcano-related words and more about this long-dormant "supervolcano." The author's first picture book reflects her own appreciation for this national treasure, which could inspire family visits. An unusual and appealing addition to the sense-of-wonder shelf. (Informational picture book. 5-8)