The first four rooms at Sweden's Treehotel, where you can sleep high in the trees of a boreal forest 50km south of the Arctic Circle, were designed to disappear into their surroundings. One, The Mirrorcube, is camouflaged by the reflections of its environment. Another, The Bird's Nest, is covered by a huge network of twigs. But the Treehotel's latest guest room, The UFO, is an even greater departure.