Not unlike the Caves of Altamira, ancient paintings and carvings by native peoples can be found in Wisconsin. According to Hidden Thunder, only 20 such rock-art sites were known in the state in 1980. Since then, nearly 200 have been discovered. Sadly, some have been vandalized, covered in thoughtless graffiti. Illustrated by photographs as well as painter Geri Schrab’s watercolor impressions, Hidden Thunder tours 11 rock art locations in Wisconsin and Minnesota. Many include images of the thunderbird, a mythic combatant against evil in the mythology of many Native American tribes. Although far from exhaustive, Hidden Thunder will serve as an entertaining introduction to a wealth of indigenous art in our area.