The GR20
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GR20

Winding some 170km from Calenzana (12km from Calvi) to Conca (22km from Porto-Vecchio), the GR20 is Corsica's most demanding long-distance footpath. Only one-third of the estimated 17,000 hikers who start it each season complete all sixteen stages (étapes), which can be covered in ten to twelve days if you're in good physical shape – if you're not, don't even think about attempting this route. Marked with red-and-white splashes of paint, it comprises a series of harsh ascents and descents, sections of which exceed 2000m and become more of a climb than a walk, with stanchions, cables and ladders driven into the rock as essential aids. The going is made tougher by the necessity of carrying a sleeping bag, all-weather kit and two or three days' food with you. That said, the rewards more than compensate. The GR20 takes in the most spectacular mountain terrain in Corsica and along the way you can spot the elusive mouflon (mountain sheep), glimpse lammergeier wheeling around the crags, and swim in ice-cold torrents and waterfalls.

The first thing you need to do before setting off is get hold of the Parc Régional's indispensable Topoguide, published by the Fédération Française de la Randonnée Pédestre, which gives a detailed description of the route, along with relevant sections of IGN contour maps, lists of refuges and other essential information. Most good bookshops in Corsica stock them, or call at the park office in Ajaccio. Still more detailed coverage of the route is featured in Rough Guides author David Abram's Trekking in Corsica book.

The route can be undertaken in either direction, but most hikers start in the north at Calenzana, tackling the most demanding étapes early on. The hardship is alleviated by extraordinary mountainscapes as you round the Cinto massif, skirt the Asco, Niolo, Tavignano and Restonica valleys, and scale the sides of Monte d'Oro and Rotondo. At Vizzavona on the main Bastia–Corte–Ajaccio road, roughly the halfway mark, you can call it a day and catch a bus or train back to the coast, or press on south across two more ranges to the needle peaks of Bavella. With much of the forest east of here blackened by fire, hikers in recent years have been leaving the GR20 at Zonza, below the Col de Bavella (served by daily buses to Ajaccio and Porto-Vecchio), and walking to the coast along the less arduous Mare a Mare Sud trail.

Accommodation along the route is provided by refuges, where, for around €9, you can take a hot shower, use an equipped kitchen and bunk down on mattresses. Usually converted bergeries, these places are staffed by wardens during the peak period (June–Sept). Advance reservation is not possible; beds are allocated on a first-come-first-served basis, so be prepared to bivouac if you arrive late. Another reason to be on the trail soon after dawn is that it allows you to break the back of the étape before 2pm, when clouds tend to bubble over the mountains and obscure the views.

The weather in the high mountains is notoriously fickle. A sunny morning doesn't necessarily mean a sunny day, and during July and August violent storms can rip across the route without warning. It's therefore essential to take good wet-weather gear with you, as well as a hat, sunblock and shades. In addition, make sure you set off on each stage with adequate food and water. At the height of the season, many refuges sell basic supplies (alimentation), but you shouldn't rely on this service; ask hikers coming from the opposite direction where their last supply stop was and plan accordingly (basic provisions are always available at the main passes of Col de Vergio, Col de Vizzavona, Col de Bavella and Col de Verde). The refuge wardens (gardiens) will be able to advise you on how much water to carry at each stage.

Finally a word of warning: each year, injured hikers have to be air-lifted to safety off remote sections of the GR20, normally because they strayed from the marked route and got lost. Occasionally, fatal accidents also occur for the same reason, so always keep the paint splashes in sight, especially if the weather closes in – don't rely purely on the many cairns that punctuate the route, as these sometimes mark more hazardous paths to high peaks.