The Plitvice Lakes in the National Park Plitvicka Jezera in Croatia. Visitors taking a cruise on lake Kozjak. The Plitvice Lakes are a string of lakes connected by waterfalls. They are in a valley, which becomes a canyon in the lower parts of the National Park. The waterfalls are formed mostly by travertine (tufa) barriers between the lakes. The lakes are listed as UNESCO World heritage and are visited by close to one million tourists per year. Europe, South Eastern Europe, Croatia