This pop-up book digs deep to unearth fascinating facts and amazing feats to entertain and educate the whole family. Two-page spreads of caves, mines, seas, tunnels, and underground systems give at-a-glance overviews as well as in-depth looks at all kinds of subterranean things, both natural and man-made. Captions throughout present facts about the deepest, largest, oldest, biggest, or most esoteric points of interest. For example, as you look under a cave flap where a bunch of bats hide, you learn that each year 20 million Mexican bats meet in Bracken Cave, Texas, to reproduce! There are lots of other surprises that are sure to delight children (skeletons, buried treasures, sharks, and rats) as well as plenty of juvenile humor (often naughty). On the underside of the main spreads, the mechanics of paper engineering are revealed: everything from brake tabs to the fulcrum to an 8-piece double-action pull-tab. These are the moving parts that dramatize the length and depth of tunnels and holes, or the speed of cars and trains, or the intricacies of escalators. This is great fun, but keep it away from tiny hands.