The Caves of Dirou (Peloponnese): No one knows how far these caves run into the seaside cliffs of Dirou on the Peloponnese's Mani peninsula. You can get some idea of their vastness by taking a tour on one of the boats that explore this underground labyrinth.

Samaria Gorge (Crete): At 18km (11 miles) long, the Samaria is the longest gorge in Europe. But although its walls reach up to 500m (1,640 ft.) and at one point are only 2m (6 1/2 ft.) apart, it's not the physical dimensions that attract thousands each year. You can credit the wildflowers, the cold stream, and the whole experience of making your way from the heart of Crete to its coast.

Santorini Caldera (Cyclades): As you approach Santorini by ferry, the whitewashed cliff-top villages of Ia and Fira resemble at first a new dusting of mountain snow. Then the ferry enters the caldera, the cliff walls rise steeply on either side, and you're suddenly enclosed in a fantastic bowl whose sides are formed of red, yellow, and ocher rock and whose surface is the shimmering blue Aegean. Some 3,600 years ago, the center of the island was blown away in a colossal volcanic eruption, leaving behind a roughly circular depression in the sea floor and the crescent-shaped sliver of rock known as Santorini. The best places to view the caldera are the walking path between Fira and Ia, and the cliff-top streets in either of the two towns.

Vikos Gorge (Western Greece): With its wooded slopes, often taxing terrain, and rugged riverbed, Vikos Gorge in Epirus is in many ways at least as impressive as the far better-known Samaria Gorge of Crete. Its remoteness means that it is also far less frequented -- and that much more a wilderness challenge.