Capri is limestone; Ischia is volcano. The two islands face each other across roughly thirty kilometres of the Bay of Naples, yet they were built by entirely different forces, and the crossing between them is a lesson in geology as much as leisure. Ischia's bulk is the collapsed cone of Monte Epomeo, uplifted green tuff riddled with thermal springs. Capri is a drowned fragment of the Sorrentine limestone ridge, pale and vertical, its Faraglioni stacks rising to 109 metres above the water. Any ischia boat tour to capri departs from one and arrives at the other, and the contrast is visible from the deck within the first half hour. The route is older than tourism. Emperor Tiberius governed Rome from Capri for the last decade of his life, from 27 to 37 AD, and the twelve villas attributed to him were supplied by boat across these same waters. Villa Jovis still stands on the island's northeastern cliff, 334 metres above the sea. Ischia, meanwhile, held Pithecusae, the earliest Greek settlement in the western Mediterranean, founded in the eighth century BC. The Nestor's Cup inscription found there remains among the oldest surviving examples of written Greek. Two islands, two civilisations, one channel between them. What sustains the crossing today is the coastline rather than the ports. Capri's perimeter is punctured by sea caves — the Grotta Azzurra, whose blue light comes from sunlight refracting through a submerged opening beneath the entrance; the Green Grotto and White Grotto along the southern shore; the Arco Naturale, a limestone arch left by a collapsed cavern. Boats trace the whole circuit in a few hours, passing Marina Piccola, the Punta Carena lighthouse and the Faraglioni arch through which small craft can pass. This is why gita in barca da Ischia a Capri remains a fixture of the Neapolitan summer, and why Capri island boat tours continue to outnumber almost every other excursion in the gulf. The island absorbed writers before it absorbed day-trippers. Norman Douglas, Axel Munthe and Maxim Gorky all wrote here. Munthe's Villa San Michele at Anacapri, built on Roman foundations, still holds his collection of antiquities above a 900-year-old stairway. An ischia boat tour to capri delivers visitors into that inheritance — a landscape shaped by emperors, exiles and erosion in roughly equal measure, and best understood from the water that separates it from everything else.