Typical for karst areas are sinkholes, crater-shaped formations that are either caused by the collapse of a cave (collapse doline) or by the dissolution of limestone by water (solution doline) — of course, both processes can play a role (cf. The Formation of Mountains). There are two particularly large sinkholes in Croatia near Imotski, a town east of Split near the border with Bosnia-Herzegovina (signposted from the Zagvozd motorway exit).
Modro Jezero (Blue Lake), on the outskirts of Imotski, is a deep hole with almost 300 m deep rock walls, filled by a blue lake whose water level fluctuates frequently. On one side the slope is less steep and here a wide path leads down to the lake. The whole thing is laid out as a park with walking paths and viewpoints.
About 1 km further along a small road you reach Crveno Jezero (Red Lake), which is, however, also blue or blue-green: only the rock walls are red here. This hole is probably one of the deepest sinkholes in the world, with the (fluctuating) water level only about halfway down to the bottom, which is 530 m below the rim and thus several metres below sea level.
Mostar and Sarajevo
Krka, Trogir, and Split
Cenotes of Yucatán
The Formation of Mountains