Eliza knows the legends about the swamp near her house -- that people have fallen into sinkholes, never to be seen again, maybe even falling to the center of the earth. As an aspiring geologist, she knows the last part is impossible. But when her best friends drag her onto the uneven ground anyway, Eliza knows to be worried.

And when the earth opens under her feet, there isn't even time to say I told you so.

As she scrambles through one cave, which leads to another, and another, Eliza finds herself in an impossible world -- where a small group of people survive underground, running from vicious creatures, eating giant bugs, and creating their own subterranean society. Eliza is grateful to be alive, but this isn't home. Is she willing to risk everything to get back to the surface?