Earle takes children on an undersea journey to explore an amazing "underwater city." She explains the formation of coral reefs and provides information about the conditions needed for survival.

In addition, the author introduces other animals that live in and near the reef and stresses the importance of protecting corals from pollution. Matthews's ink-and-gouache paintings bring the colours and variety of this habitat to life. The book also includes a simple experiment that demonstrates how "filter feeders" strain food from the water.