Wikipedia Reference: Ariel

Ariel does not appear in Ilium. According to Savi, Ariel is the sentient biosphere of Earth, and co-creator of the Calibani. Savi claims to have made a pact with Ariel, allowing passage across the Mediterranean Basin. Ariel has the presumed ability to move between the e-ring and Earth, as Prospero says that it is Ariel who freed the sonie, which was trapped in Jerusalem.