Just south of the national park, near Šavnik, is the remarkable 2.7km-long Nevidio Canyon. Cut by the Komarnica River, at points it is only metres wide, hence the name (nevidio means 'invisible'). It's extremely beautiful but equally dangerous. Canyoning expeditions generally take about three to four hours and participants should be able to swim and have a high level of fitness.
July and August are the safest months to explore, and then only in the company of professional guides. Montenegro Canyoning is a highly recommended group that focuses solely on expeditions to Nevidio. Otherwise, Anitra/Grab (www.tara-grab.com) organises expeditions out of Nikšić (price on application), and Summit Travel Agency and Durmitor Adventure do so out of Žabljak (€100 per person including lunch, minimum two people).