Veliko Jezero (Mljet)

Veliko Jezero, the Great Lake, is the larger of the two saltwater lakes in the north-western corner of Mljet, and it behaves less like a lake than like a landlocked bay with its own tide. Water moves in and out through a single narrow channel, which makes the interior calm, a few degrees warmer than the open Adriatic, and home to a coral colony and a jellyfish found nowhere else. For anyone arriving by boat it is one of the more unusual pieces of water in Croatia, and one of the few worth planning an approach around rather than simply motoring into.

A Drowned Karst Basin, Not a Lake

Veliko Jezero covers roughly 145 hectares, about 1.44 square kilometres, and reaches a maximum depth of around 46 metres, divided into three basins. Its smaller neighbour, Malo Jezero, covers about 24 hectares and drops to roughly 29 metres. Neither is a lake in the geological sense. Both are drowned karst depressions, dry or freshwater basins during the last glacial period, flooded by the sea as levels rose after the ice retreated. They were probably still fresh somewhere between 10,000 and 6,000 years ago, before seawater worked its way in through fissures in the limestone and the low ground at Soline.

The link to the open sea is the Soline channel, and its dimensions explain almost everything else about the lake. The channel is only about 10 metres wide and roughly 2.5 metres deep, so the entire tidal exchange for a 145-hectare basin is forced through a gap narrower than most harbour entrances. The result is a real current running in and out of the lake, strong enough to matter to a slow boat or to a swimmer who has drifted too close to the throat. It also means the water turns over extremely slowly: one long-standing estimate puts the residence time of water inside the lake somewhere between 8.5 and 21.5 years. Salinity sits slightly below that of the open Adriatic because of freshwater seeping in, and the enclosed body warms faster in summer than the sea outside it.

Speed inside both the channel and the lake is capped at 5 knots for every vessel, park authority boats included. That limit is not decorative. The combination of a tight entrance, tidal flow and swimmers in the water leaves no margin at all.

The Largest Coral Bank of Its Kind

Close to the channel entrance, on the bed of Veliko Jezero between roughly 6 and 18 metres down, lies a bank of Cladocora caespitosa covering more than 650 square metres. It is the largest bank of this coral recorded anywhere to date. Cladocora caespitosa is a Mediterranean endemic and the sea's only reef-building coral that hosts symbiotic algae the way tropical corals do, which makes it a useful reference organism for studying how such corals respond to warming water. Researchers link the size of the Mljet colony to the same tidal currents through Soline: the steady bottom flow appears to favour growth and the accretion of the bank. The species is classed as endangered, and this colony has been surveyed repeatedly over the past two decades to track the effects of marine heat waves and coastal pressure.

The lake's other biological curiosity is its jellyfish. Aurelia relicta was formally described as a separate species in 2016, and so far it is known only from here; the specific name comes from the Latin for abandoned or forsaken, a reference to its relict isolation. It is a moon jellyfish relative that grows conspicuously large, up to around 55 centimetres across, and it is harmless to swimmers. Snorkellers looking for the coral tend to run into it, along with sponges and small fish, on the calmer inner shores.

St Mary's Isle and the Benedictines

The islet in the middle of the lake, barely half a hectare in extent, carries the best-known building on Mljet. Benedictine monks from the house of St Mary at Pulsano on Monte Gargano, in Apulia, were invited to the island by Desa, ruler of Zahumlje and Duklja, in 1151. The monastery and its church went up between 1177 and 1198, and Pope Innocent III issued the document consecrating the church of St Mary in 1198.

The church is a single-nave Romanesque building in the Apulian manner, modelled on the mother house at Monte Gargano. It was altered in the 16th century, when the porch received the coat of arms of the Gundulic family and a defensive tower was added at the south-eastern corner, turning the complex into something that could be held against a raid. The Benedictines remained until 1809, when the community was suppressed under Napoleonic rule. The buildings then passed to the state and declined; between 1960 and 1991 the complex operated as a hotel, and in 1998 it was returned to the Diocese of Dubrovnik, since when restoration has proceeded in stages.

Reaching the Lake

Veliko Jezero sits inside Mljet National Park, proclaimed on 11 November 1960 and generally described as the oldest marine protected area in the Mediterranean. The park covers close to 5,300 hectares including a belt of sea 500 metres out from the coast, and takes in Malo Jezero, Soline bay and the villages of Pomena, Polace and Govedari. Mljet itself is about 37 kilometres long and 98 square kilometres in area, with just over a thousand permanent residents; the catamaran from Dubrovnik takes about 90 minutes and the car ferry around two and a half hours.

Arriving by boat means timing the Soline passage rather than simply motoring through, so check which way the current is running before committing, particularly with a heavy or underpowered vessel. Once inside, the calm water, the warmth and the coral make Veliko Jezero a popular stop for gulet charter guests exploring Dubrovnik and the islands further down the coast.