The Dragonriders of Pern - Anne McCaffrey [1]
It took generations to develop to the fullest the potential of these dragons. The second phase of the proposed defense against the deadly incursions would take even longer. For Thread, a space-traveling mycorrhizoid spore, with mindless voracity devoured all organic matter and, once grounded, burrowed and proliferated with terrifying speed. So a symbiote of the same strain was developed to counter this parasite, and the resulting grub was introduced into the soil of the southern continent. The original plan was that the dragons would be a visible protection, charring Thread while it was still skyborne and protecting the dwellings and the livestock of the colonists. The grub-symbiote would protect vegetation by devouring any Thread that managed to evade the dragons’ fire.
The originators of the two-stage defense did not allow for change or for hard geological fact. The southern continent, overtly more attractive than the harsher northern land, proved unstable and the entire colony was eventually forced to move north to seek refuge from the Threads in the natural caves on the continental shield rock of the north.
The original Fort, constructed in the eastern face of the Great West Mountain Range, soon grew too small to hold the colonists. Another settlement was started slightly to the north, alongside a great lake conveniently formed near a cave-filled cliff. But Ruatha Hold, as the settlement was called, became overcrowded within a few generations.
Since the Red Star rose in the east, the people of Pern decided to establish a holding in the eastern mountains, provided a suitable cavesite could be found. Only solid rock and metal, both of which were in distressingly short supply on Pern, were impervious to the burning score of Thread.
The winged, tailed, fire-breathing dragons had by then been bred to a size that required more spacious accommodations than the cliffside holds could provide. But ancient cave-pocked cones of extinct volcanoes, one high above the first Fort, the other in the Benden mountains, proved to be adequate and required only a few improvements to be made habitable. However, such projects took the last of the fuel for the great stonecutters, which had been programed only for regular mining operations, not for wholesale cliff excavations. Subsequent holds and weyrs had to be hand-hewn.
The dragons and their riders in their high places and the people in their cave holds went about their separate tasks, and each developed habits that became custom, which solidified into tradition as incontrovertible as law.
Then came an interval of two hundred Turns of the planet Pern around its primary—when the Red Star was at the other end of its erratic orbit, a frozen, lonely captive. No Thread fell on Pern. The inhabitants erased the depredations of Thread and grew crops, planted orchards from precious seed brought with them, thought of reforestry for the slopes denuded by Thread. They even managed to forget that they had once been in grave danger of extinction. Then the Threads fell again when the wandering planet returned for another orbit around Pern, bringing fifty years of attack from the skies. The Pernese once again thanked their ancestors, now many generations removed, for providing the dragons who seared the dropping Thread midair