After a record-breaking storm, David and Chuck discover half-million-year-old fossils on the cliffs where no such bones should be, prompting Tyco Bass to reveal some of the history and customs of the Mycetians, the Mushroom People of Earth. After Tyco's departure, the boys' discovery of the ailing Prewytt Brumblydge's unexpected Mycetian connections leads them to attempt a new, unscheduled trip to Basidium. Illustrated by Leonard Shortall.