El Teide, Tenerife

The highest peak in Spain, this ancient — but still simmering — volcano is also one of Europe's top natural wonders. The Pico de Teide and the Caldera de las Cañadas, a gigantic volcanic crater, together form the Parque Nacional del Teide, at the center of the island of Tenerife. In listing the park in 2007, UNESCO cited its natural beauty and "its importance in providing evidence of the geological processes that underpin the evolution of oceanic islands."

You can explore El Teide in several ways. You can drive or hike across the inside of the caldera — the crater floor — 12 miles in diameter and a barren moonscape of colored rock formations that's like driving into the center of the earth. You can climb El Teide's cone, but an easier way to get close to the top is by an eight-minute cable car ride. On a clear day, views cover the entire archipelago and can extend to North Africa — the nearest land mass to the Canary Islands.