Port-Cros
Europe's oldest marine national park — two of the Îles d'Hyères with their underwater belts, Posidonia meadows, groupers, and forts in the maquis.
Port-Cros was declared in December 1963, five months after the Vanoise, and took the idea somewhere no European country had yet gone: below the waterline. The park covers the middle island of the Îles d'Hyères with a 600-metre belt of sea around it — the first marine national park in Europe — plus the islet of Bagaud, the rocks of Gabinière and Rascas, and, since the park's 2012 refoundation, the state-owned parts of neighbouring Porquerolles and its waters. The island itself is small enough to walk end to end before lunch: five square kilometres of holm oak, Aleppo pine and hard green maquis, without cars and nearly without permanent residents.
The sea is the point. Posidonia oceanica — the slow-growing seagrass that is the Mediterranean's nursery, oxygen plant and beach-builder — meadows the shallows, and sixty years of protection have restored what fishing pressure stripped elsewhere: dusky groupers hang in the water column off La Gabinière like sandbagged zeppelins, with barracuda, corb and bream in schools that follow snorkellers around. A marked underwater trail off La Palud beach makes the introduction; divers get the deeper ledges. Ashore, the island's military centuries left five forts — Estissac and the Fort du Moulin can be visited — and the wooded valleys carry names like Vallon de la Solitude without irony. Yelkouan shearwaters breed on the cliffs, and the Tyrrhenian painted frog, a relict amphibian shared with Corsica and Sardinia, persists in the island's few damp places. Bagaud is an integral reserve: no landing at all.
What's worth knowing before you go
Boats run year-round from Hyères and seasonally from Le Lavandou; the crossing is short, the last return boat is earlier than you think, and July–August day-trippers fill the Plage de la Palud by late morning. In high summer, fire risk closes the interior trails on red-flag days — check the daily massif bulletin before crossing. There is one village, a handful of restaurants, and no camping; water is scarce and the island takes that seriously.
The connoisseur's season is September: the sea is at its warmest and clearest, the groupers at La Gabinière are still resident, and the boat back to Hyères is half empty.