The smallest of the four inhabited islands in Spain’s Balearic archipelago is Formentera, 13.5 miles long and just over a mile wide. With no airport — the only way to get here is by boat from neighboring Ibiza — Formentera is also the least touristed, with a laid-back air rare in the Mediterranean.

But it’s not unknown to divers, who treasure its crystal-clear waters. With visibility reaching as far as 150 feet, the sea surrounding its beaches and cliff-lined shore is nothing short of spectacular. This marine landscape harbors the best-preserved posidonia fields in the entire Mediterranean, and this plant, also known as Neptune grass, provides an ideal habitat for multiple species of fish and other marine wildlife.

Feathery sea-fans called gorgonians adorn underwater cliffs and caves, and off Punta Pedrera the sunken shipwreck of the Florin Brothers lies less than 100 feet deep against a wall of sea grass teeming with fish. Elsewhere, La Plataforma, formerly a floating fish farm for gilthead bream, lies between 30 and 100 feet of water in an undersea landscape filled with fish.

In all, about 30 dive sites lie around Formentera, most of them inside the Es Freus-Ses Salines Nature Reserve. This area, declared a natural park in 2001, covers most of the southern tip of Ibiza and northern Formentera, as well as the islets between them. The park’s shoreline salt marshes protect 210 bird species, including herons and flamingos.