Red and Blue lake
The sinkholes of Crveno Jezero (Red Lake) and Modro Jezoro (Blue Lake) are located only a few hundred meters apart in the Dalmatian province of eastern Croatia, near the border with Bosnia-Herzegovina. The sinkholes formed by underground rivers penetrating the Cretaceous limestone above and the dolomite below. The area contains around 18 small and big water filled depressions.
Crveno Jezero is nearly circular and 528m deep with a diameter of 450-500m making it the third deepest sinkhole in the world. It is filled with dark blue water to a depth of 290m and contains 16 million cubic m of water. The depth of the lake was confirmed by an expedition of speleodivers in 1998. At the bottom of the lake a cave canal is orientated to the southwest. However, this cave canal has not been mapped yet. Actually quite a number of caves are present near the water level of the lake.
Locally, Crveno Jezero is known as Red Lake which is not due to the red color of the water (it is actually quite blue) but to the red cliffs that surround the sinkhole. The redness of the cliffs is caused by the presence of iron oxides, an element of the common soiltype known as Terra Rossa which is found throughout the Mediterranean. The cliff walls are almost vertical except for one side which collapsed during an earthquake in 1942. Because of its depth and the dangers of collapsing cliff walls the lake is not accessible (not mentioning that it is a daredevil climb down to even get there although you could also base jump). Crveno Jezero is also home to the endangered species of fish called Delminichthys adspersus. This small fish (12cm) are solely found in the Dinaric Alps of Croatia, they require extremely clear water which is found in this region commonly. Sometimes the fish are found in springs surrounding the sinkhole which indicates that underground the sinkhole is connected to other water bodies.
Only a few hundred meters next to Red Lake is Modro Jezero known as Blue lake due to its intense blue-green color. Unlike the almost perfectly circular Red Lake Blue Lake is oblong. The lake is up to 90m deep but is known to disappear completely at the end of summer. Overall the Blue lake sinkhole reaches a depth of 290m from top to bottom. Here, the 1942 earthquake also caused a large landfall which reduced the depth of the lake. Unlike Red Lake Blue Lake is a popular place for swimming since it is not as deep and easily accessible. The appearance of a boulder in the lake, named Jakićin after a boy who drowned here in 1834 usually marks the beginning of the swimming season. When the lake dries up in the summer local soccer matches are played in the middle. The southern nearly vertical rim is often used for cliff jumping (into the water ). As with Blue lake the sinkhole is surrounded by other small caves. The 9th century fortress of Imotski towers above the high cliffs. Hiking trails around the lake were build in 1907 under reign of the Habsburg emperor Franz-Josef.
--BO
Image: Courtesy of www.travelcroatia.com. Red Lake seen from above.
References:
http://www.wondermondo.com/Countries/E/Croatia/SplitDalmatia/CrvenoJezero.htm Here you can see a cross section of the cave as compared to a large airplane.
http://www.cavernas.org.br/anais26cbe/26CBE_555-559.pdf
http://www.wondermondo.com/Countries/E/Croatia/SplitDalmatia/ModroJezero.htm