In Heretics of Dune, all life on Arrakis is destroyed (and the entire surface of the planet slagged into oblivion) by the Honored Matres in a failed attempt to eliminate the latest Duncan Idaho ghola. The Bene Gesserit escape with a single sandworm, and drown it to revert the worm back into sandtrout. In Chapterhouse: Dune the Bene Gesserit use these sandtrout to begin a new sandworm cycle on their homeworld of Chapterhouse, which is terraformed into desert for this purpose.
