During the Second Long Interval, five weyrs (Fort, High Reaches, Igen, Ista, and Telgar) were empty. All the inhabitants, Dragons, Riders, and support staff, had mysteriously vanished shortly after the end of the Eighth Pass.
At the time, this came as a major shock to the rest of Pern. No one could figure out why every last person and dragon in those five weyrs had disappeared, nor why Benden had been spared whatever unknown calamity struck the other five.
Over time, the shock faded, and the people came to forget their duties to the Weyrs, and the question of what happened to the Weyrs faded out of conciousness. Many believed that Thread had vanished forever.
When Thread returned, Pern was panicstruck. It soon became clear to the remaining Dragonriders (though they were careful to hide this fact from others) that they were too few to fight Thread.
Then Lessa discovered the truth. The Dragons had gone ahead ... 450 Turns to the present day. She travelled back in time to describe the predicament that Pern found itself in, and to request help. Benden had to remain, in order that Lessa could become its Weyrwoman and travel back in time, but the rest went forward to the Ninth Pass, reoccupying their abandoned Weyrs, 1800 fighting dragons.