From Split: Plitvice Lakes National Park Guided Tour

Plitvice are known as a natural phenomenon because they keep changing through sedimentation of calcium carbonate and create special kinds of algae and moss, tufa or travertine which has been deposited and is still deposited to form the natural dams that separate the lakes. Since the process is ongoing today, just as it always has, new travertine barriers, curtains, stalactites, channels, and cascades are built as the existing ones keep changing. So it doesn’t matter if you have visited the lakes before, they probably look different now, so you can visit them again and never be bored. Besides the lakes, there are some interesting caves in which remains of prehistoric settlements have been found.