Jk Rowling own the Harry Potter universe along with Warner Bros. Raincoast & Schoolastic Books, etcetera as to the owners of The Dragonriders of Pern.
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Harry Potter had moved most of the dragons and dragon riders off earth to Pern
One of the riders had an image in his mind, only it was not of earth but Pern. And when the rider was going to go a flight to test out a new product he was thinking how nice it would be to go to Pern and his dragon saw the images in his partners mind so he went in between and they appeared in the skys over Pern
His dragon was making joyous sounds as he landed near a cave. The sky was filling with thread and so they needed to get out from under it.So they sought shelter in the cave.
His rider glen not know his dragon had sent a telepathic message to Harry's dragon as to where they ended up and as to what was occurring.
As unlikely as it was Harry and several dragon riders with their dragons went between to Pern a d they set off a couple tons of ANFO in the skies and went between to the cave.
As the ANFO went off it cleared and area of sky in size to Hiroshima,Japan, All the dragon riders snd there dragon flew out of the cave into a clear sky. They took some samples of the thread, then went between back to earth.
Sir Robert had the thread samples checked out to discover what it was exactly and the best way to destroy it.