In a country not particularly noted for ecological foresight, Europe's largest marine park (2260 sq km) is a welcome innovation. Created in 1992, its prime aim is to protect the endangered Mediterranean monk seal and several rare seabirds. In summer, boats from Alonnisos and Skopelos run full-day trips through the pristine park, whose sea floors are carpeted in oxygen-producing posidonia. The shy monk seal is rarely seen, but you may spot dolphins (three species), turtles, Eleonora's falcons or migrating whales.
The marine park is divided into two zones, A and B. Alonnisos lies within Zone B, along with the islets of Peristera and Dio Adelphi. Access to Zone A is more restricted, with boats allowed no closer than 400m to most islets; one exception is the island of Kyra Panagia, whose beautifully renovated monastery dating from 1200 is visited on most boat trips. Rocky Piperi islet is protected by a 5km radius due to its importance for monk-seal reproduction and raptor populations.