Mljet is the greenest island in the Adriatic, so thickly pined that Croatians say so outright. At its heart lie two saltwater lakes, with a twelfth-century Benedictine monastery standing on an islet in the larger one. An island on a lake on an island is the picture that has come to stand for Mljet. The lakes are Veliko and Malo Jezero, the Great and the Small, joined to each other and to the sea by narrow channels. A tidal current pours through them, reversing direction every six hours as the sea breathes in and out. They held fresh water until Roman times, when the Adriatic worked its way in through a crack in the limestone. In 1960 the whole western third of the island became Mljet National Park, the oldest marine protected area in the Mediterranean.
No cars are allowed in the park, so the quiet is almost complete — the dip of an oar, wind moving through the pines, the odd blackbird. From the little harbours of Pomena and Polače you can walk or cycle the shaded path that rings the Great Lake, stopping to swim off warm slabs of rock where the water runs clear to the bottom. A shuttle boat crosses to the monastery islet, where you can drink coffee in a courtyard the Benedictine monks once ran as Croatia's first quarantine station. Mornings leave the lakes flat as glass; by afternoon a breeze lifts and small sailboats slip into the bays. Reaching Mljet still takes some effort — a ferry or fast catamaran from Dubrovnik, or the car ferry from the Pelješac peninsula — and that distance keeps it calmer than the islands closer to the mainland.