Cumae, founded by the Greek city of Chalcis in c. 750 BCE, lies in Campania, on the coast, near Naples. Powerful in its early days, by Juvenal's time it was known primarily as the home of the ancient Sibyl who legendarily sold her books of prophecy to the Roman King Tarquinius Priscus (see Pliny, Historia Naturalis 13.88), and as the entrance to the underworld.
There is a certain irony in the "secession" or "retirement" of a cosmopolite to the rural countryside. By relocating in Cumae, Umbricius becomes a foreigner twice over: he reverses the typical pattern of country dwellers migrating to the city and he moves into a municipium which was founded by the very Greeks he criticizes. Juvenal's approval, however, rests on population numbers: Umbricius' departure from densely packed Rome will be barely noticed, while his entry into sparsely occupied Cumae will be a source of pride ("one inhabitant to boast").