Mljet is the greenest island in the Croatian Adriatic, a long ridge of pine south of Dubrovnik, and its western third is a national park built around two lakes that are not fresh water at all. The Adriatic found a crack in the limestone, came in and never got back out. The Great Lake and the Small Lake still fill and empty with the tide through a narrow neck, where the current runs like a river and turns around every six hours. A flat path rings the Great Lake, shaded almost the whole way by Aleppo pines leaning out over the water. You walk or cycle it in a couple of hours, dropping onto the warm rock slabs to swim wherever the trees open. Kayaks go out onto the sheltered arms, where the colour shifts from deep blue to pale green over the bright limestone floor, and the crossing into the Small Lake goes easiest near slack water.
A shuttle boat carries you over the Great Lake to the islet of St Mary. Benedictine monks from Puglia were given it in 1151, built their monastery over Roman ruins and held the island for more than six centuries. They were turned out under Napoleon in 1808. A café works out of the rooms they lived in, and the coffee is ordinary; what you are there for is a stone courtyard on an islet in the middle of a saltwater lake. On the wild south coast a sea cave has collapsed open at the waterline, and islanders hold it to be Calypso's, where the nymph kept Odysseus for seven years. You swim into it on a calm day, when the light comes in under the surface and the whole cavern goes electric blue. Not everything that arrived here was invited. In 1910 the Austro-Hungarian authorities let Indian mongooses loose to clear the island of its horned vipers, and within twenty years the snakes had all but gone. The mongooses moved on to the birds and lizards, and one still crosses the shore path ahead of you.