Treehotel owners Britta Jonsson Lindvall and Kent Lindvall enlisted six designers to create the 13-by-13-foot Mirrorcube, a reflective glass cube built around the trunk of a pine. It blends into the surrounding forest so well that the architects plan to cover it with a transparent ultraviolet film to alert flying birds, so they won't smash into it. Click here to see the interior and other modern tree house designs.