Complex sentence: Creek Mostarska Bijela forms a unique krastic formation also known as sinking river, draining its waters underground, What's unique in case of Mostarska Bijela is that part of underground flows is traversable.
Split into simpler sentences: Creek Mostarska Bijela forms a unique krastic formation also known as sinking river, draining waters of one section of it flows underground. What is unique in case of Mostarska Bijela is that part of this underground section is traversable.