Mariana is a third grader living in Las Cruces, New Mexico at the northern fringe of the Chihuahuan Desert. One day her class visits Chihuahuan Desert Nature Park to hike to the top of an extinct volcano and see the plant and animal life that is special to this desert. Led by Dr. Whitford, the nature park scientist, the children are challenged to find as many signs of animals living along the trail as they can. Mariana is amazed at all the life around her that was difficult to see at first glance. Adapting to the desert environment, many animals were camouflaged or sought shelter from the sun in burrows. At last atop the volcano, Dr. Whitford tells the children that This is a great example of an ecosystem that has shifted. Once grassland, the landscape is now mostly covered in shrubs like creosote bush and mesquite. Suddenly a storm comes up with little warning, and the children rush back to their bus. As the rain subsides the bus driver delivers the unwelcome news that the bus will have to be pulled out of the now muddy road. While waiting for help the children get out of the bus and head for a nearby Playa, a special kind of shallow lake that is formed after a heavy rain. As Mariana wades through the Playa she notices small animals in the water, and scoops one up. Dr. Whitford identifies it as a tadpole shrimp, animals that hatch only when water fills the lakes. At last the bus is pulled out of the mud by a rancher's tractor, and Mariana returns home to write to her friends about her day in the desert.