Republika Srpska and the Federation of Bosnia and Herzegovina are the two entities of Bosnia and Herzegovina. Thats strange and a result of the Bosnian War. However, this area is more or less the northern part of the country and it is part of the Dinarides and heavily karstified. There is a watershed, and despite being so close to the Mediterranen Sea the region actually drains to the Black Sea.
The region has numerous exceptional karst feature and great caves. The Trebišnjica is one of the longest sinking rivers in the world. It belongs to the Adriatic Sea drainage basin. There are two show caves but both are actually closed.