Fifty kilometres off the coast of Castellón, in the open Mediterranean, sits a ring of rock that from the outside looks like half an island. It is not half. It is what remains of a volcano's crater - and the boat that takes you there sails literally inside it.

The Columbretes are four groups of islands: Illa Grossa, la Ferrera, la Foradada and el Carallot. Together they hold nineteen hectares of land. The caldera is almost 800 metres across, and the mouth of the bay you enter through is 340 metres. The volcanic activity that created all of it is between 1.3 million and 300,000 years old, and the underwater volcanic field beneath them stretches 90 by 40 kilometres, at depths of 80 to 90 metres.

Only one island is open, and even then with a quota

Landing is permitted only on Illa Grossa, with a daily visitor quota and rationed climbs approved by the regional authority. That is not a tourist gimmick for the sake of exclusivity - it is the reason the place still looks the way it does.

The Greeks and Romans called them Ophiusa and Colubraria - the islands of snakes, because they were literally full of them.

The lighthouse and the solitude

Construction of the lighthouse began in October 1856 and it entered service three years later, with French optics that cast light as far as 21 nautical miles. People lived there for decades - until 1975, when solar panels made a permanent human presence unnecessary. That is when the island emptied.

What lies under the water

Since 1988 the Columbretes have been a nature park, the first protected area in the province of Castellón, and later a marine reserve with additional European protections. The result of almost forty years of protection is immediately visible: the seabeds are among the best preserved in the Mediterranean, and large groupers swim past divers without fleeing. Fish that do not flee from humans is the most direct indicator that nobody has fished there in decades.

On land lives the Columbretes lizard, an endemic species with a body of five to six centimetres. Above it - Eleonora's falcon, Audouin's gull, the storm petrel and the cormorant.

Why this is worth knowing

This island is proof that a quota works. Fifty kilometres from shore, not one hotel, a daily limit on people - and after four decades the sea beneath it is among the healthiest in the whole Mediterranean. Does anyone here, on Lake Ohrid or Lake Prespa, even open that account when it comes to visitor numbers?