Saint Lucia's twin peaks are deceptive. Based on their names, Petit Piton should be a quick and easy mini-climb, while Gros Piton should be a bear of an ascension.

Wrong.

Petit Piton is the Everest of this 32-square-mile Caribbean island, requiring ropes, harnesses, and rock-climbing wherewithal. Meanwhile, Gros Piton is an arduous yet technique-free romp up the mountain. Perhaps they should switch names.

Most likely, they won't. The volcanic marvels are island icons—the local beer is called Piton, and most postcards brandish the sister peaks—as well as a UNESCO World Heritage Site (inscribed 2004). The pair burst from the sea like giant lumps of earth, dominating the southwestern coast and the tiny town of Soufriere. From below, the cone-shape mountains look impenetrable, with barely a break in the thick vegetation, yet they're not; with a guide, you can go from bottom to top and back, and live to tell.

Petit Piton is 159 feet shy of Gros Piton—2,460 to 2,619 feet. But what the smaller peak lacks in height, it makes up in difficulty. The lower segments require only a sturdy pair of hiking boots, but the upper portion demands belaying skills and a quicksilver touch. Only climbers accompanied by an experienced guide should tackle the mountain, and should dedicate at least four and a half hours for the up-and-back trip. In addition, officials try to limit the number of climbers to protect the vital mountainous environment. At risk are 97 plant species on Petit Piton alone, including eight rare tree specimens; 27 bird species, of which five are endemic; indigenous rodents; bats, amphibians and reptiles; and one type of opossum.

Compared to the more challenging Petit adventure, Gros Piton is a cake walk—though it still twists and torques the muscles, and will swallow half a day of your itinerary. The expedition starts in the car, which traverses low rushing rivers and rumbles over flinty terrain on the way to the trail head.