 McCaffrey's best-known works are the "Dragonriders of Pern" series.After colonists from Earth make a decades-long journey to a new planet (which they name 'Pern', from the report of the initial exploration of the planet.Pern is an acronym for "Parallels Earth, Resources Negligible"), a volcanic eruption and an extra-planetary threat called "Thread" force the original colonists to retreat into caves in the northern hemisphere of the planet, abandoning most of their advanced technology in the process.