The Cicerone Corisca guidebook to walking the GR20, is known as the toughest waymarked trail in Europe. It makes a complete traverse through the high mountains, sometimes with hands-on scrambling. Covering 190km, it can be completed by fit walkers in about two weeks. The guide includes high- and low-level alternatives, plus extra mountain climbs. Ideal for mid-May to mid-October; June or September is ideal, with July and August being the hottest, and busiest, months.