Meet NASA inventor Penny Boston! She's here to introduce readers to Martian cave colonies. Alongside Penny, readers will learn about astrobiology and the possibilities of living in caves on Mars. They'll explore caves throughout the solar system, microbes, inflatable cave liners, bioreactors, and oxygen. The Inventor Challenge at the back of the book encourages readers to create their own inventions. For more information about interesting space inventions, check out other titles in the Out of This World series!.