Shell World (Polish Świat Muszli) is the third novel in the Expedition series. It was written by Polish author Ewa Karwan-Jastrzębska under the pen name Eva Carvani and has illustrations by Daniela Vetro, Paolo Campinoti and Mara Damiani. It was first published in Poland in 2008 by Egmont.
In this novel, the W.I.T.C.H. girls team up with the witches on Island Reef. Will, Irma, Taranee, Cornelia and Hay Lin learn that in the surroundings of the island often they see the waters form strange eddies, and in the depths of the ocean ships disappear. The girls decide to explore the depths, and on the way, they uncover the terrible truth about the Other World. Mysterious lighthouse, old map, the magic triangle, weird fish are just some of the puzzles that the girls will have to unravel.