A floating barrier does not block the entrance arbitrarily: this long sea cave brings together a sensitive habitat, exhaust fumes, impact marks, swimmers and a natural rockfall risk.
The barrier is back after strong jugo winds
The ranger and specialist service of the More i krš public protected-area authority has reinstalled the floating barrier in front of Medvidina Cave on Biševo. Strong jugo winds had previously torn it away. The line is intended to prevent motor-powered boats from entering; according to the authority, the corresponding ban was ordered by the nature-protection inspectorate.
The barrier is therefore neither a decorative swimming line nor something to be moved aside for a better view. It makes a rule that is easy to overlook visible in the water: motor-powered travel ends in front of the dark cave entrance.
A 160-metre sea cave, not an open rock tunnel
Medvidina Cave lies in Trešjavac Bay on the southern coast of Biševo. Its monumental entrance is approximately 27 metres high and 14 metres wide. Beyond it, the cave narrows and its ceiling becomes lower; after about 160 metres, it ends at a small pebble beach. The cave has been protected as a geomorphological natural monument since 1967.
From a boat, the large entrance initially appears spacious. That perspective is deceptive: inside, manoeuvring room, light and air exchange decrease. A vessel that can turn without difficulty outside becomes a loud, exhaust-producing mass that is hard to avoid within the cave.
Impact marks and exhaust fumes reveal the direct damage
The protected-area authority reports visible impact marks on the cave walls. It also mentions engine exhaust that can accumulate in the enclosed rear section and harm the habitat there. Both are direct consequences of a use that is incompatible with protecting a confined natural environment.
The issue is therefore not merely noise or whether a skipper proceeds politely and slowly. Even a carefully handled motorboat releases exhaust into a poorly ventilated space and can strike rock because of a wave, gust or small steering error. A clear boundary outside is more reliable than hoping every skipper will react in time once inside.
Biševo · Medvidina Cave
What the floating line protects
Boats and swimmers cannot share a safe corridor inside the cave
The authority explicitly warns against boats and swimmers being in the cave at the same time. People in dark water are difficult to see from the helm, while there is little room to move aside. In this setting, a propeller, hull, exhaust and wake become hazards more quickly than they would in an open bay.
The barrier resolves this conflict on the boating side: motor vessels remain outside. This does not, however, amount to an official invitation to swim in. Any visit is undertaken at one's own risk and must take account of the weather, sea state, visibility, fitness, return route and instructions on site.
A natural rockfall risk remains above the entrance
As early as 2018, Geopark Vis and the Croatian Mountain Rescue Service HGSS examined the area after rock had broken away above the entrance. At the time, they found no point in immediate danger of collapse. Their expert assessment nevertheless found that the heavily karstified rocks around the entrance could shed further pieces.
The geopark therefore advised caution and recommended avoiding a visit for safety reasons; anyone who went there nevertheless did so at their own risk. The current notice from the protected-area authority also highlights the possibility of rockfall in front of the entrance. An intact buoy line can stop motorboats, but it cannot prevent a natural rockfall.
The name recalls the Mediterranean monk seal
At the end of the cave is a small beach historically documented as a resting and breeding site for the Mediterranean monk seal. The cave takes its Croatian name, Medvidina špilja, from this association. It is regarded as a former habitat, and one that could potentially become suitable again, for one of the world's rarest seal species.
That is not a promise that visitors will encounter a seal there today. Particularly in an area to which the species might return, freedom from disturbance matters more than a spectacular sighting attempt. Light, noise, litter and pushing on as far as the beach at the rear are at odds with the aim of preserving the protected cave as a habitat.
How to behave correctly at the protective line
Motorboats must not enter the cave or moor to the barrier. The installed infrastructure must not be opened, crossed or damaged. Visitors should avoid leaving waste, refrain from disturbing animals, and respect information boards and instructions from the competent authorities.
Skippers should keep enough distance to prevent wind and waves from pushing the boat onto swimmers or buoys. Bathers arriving from an excursion boat should clarify the return route, pick-up point and conditions with the crew. If jugo winds, swell or poor visibility develop, turning back makes a better story than forcing a cave visit.
Protection is intended to extend beyond a single barrier
Within the Interreg ASPEH project, the Blue, Green and Medvidina caves in the Vis archipelago serve as a pilot area for protecting submerged or partly submerged sea caves. Plans include conservation and visitor-management schemes, a possible limit on visitor numbers, and trials involving lower-emission or electric propulsion.
This shows the direction of travel: the floating line outside Medvidina is a specific immediate measure, but it is not the whole management strategy. Lasting protection requires rules, robust infrastructure, enforcement, clear information and ways to experience nature without motor exhaust inside the cave.
Evidence
Sources & date
Sources support the facts. Planning notes and recommendations are Golden Beach editorial assessments.
- Public Notice: Rules of Conduct in Medvidina Cave on BiševoVis Archipelago Geopark, based on JU More i krš · June 17, 2026Motorboat ban, protective infrastructure, rules of conduct and the link to the ASPEH conservation plan
- Floating barrier reinstated in front of Medvidina CaveMorski.hr, based on JU More i krš · August 24, 2026Reinstallation after strong jugo winds, inspectorate order, impact marks, exhaust, swimmers and rockfall warning
- Medvidina CaveJavna ustanova More i krš · Current protected-area information, checked August 25, 2026Formation, protected status, cave form and significance as a potential Mediterranean monk seal habitat
- Protected natural monuments and significant landscapesVis Archipelago Geopark · Checked August 25, 2026Location, entrance dimensions, length and rules in the rear section of the cave
- Public statement – Medvidina CaveVis Archipelago Geopark · August 14, 2018Joint rock inspection with HGSS, remaining rockfall risk and visits at one's own risk
- 2025 annual financial report – ASPEHJavna ustanova More i krš · March 2026Three-cave pilot area, visitor management and planned electric propulsion
