Located 30 miles (50 kilometers) south from Nalchik, Russia lays the Blue Lake. It is believed to be one of the deepest lakes in the world at 846 feet (258 meters) deep. Although deep it is not a large lake at 770 feet by 426 ft. It is oval shaped shaft the burrows deep into the Earth. With unexplored caves it may be even deeper. As a comparison Lake Michigan measures a vast 307 miles by 118 miles and a maximum depth of 925 feet.
No rivers or streams seem to feed this body of water. Not that can be seen. The lake is maintained by an underground source. While cave fed the water is extremely clear; visibility can reach up to 131 feet depending on where you are in the lake.
Like a blue sapphire.
If you were to look at the lake it would look like a sapphire surrounded by trees. The water color ranges from azure to emerald depending on lighting and depth. These color variations are caused by hydrogen sulfide and algae. Large areas of the lake floor appear to be covered in a green carpet all.
What is that smell?
The lake does not smell pleasant because of the hydrogen sulfide. The locals nick named it the stinky lake. It has a scent of rotten eggs. If you have ever smelt a bad egg you can imagine the smell here. With the presents of hydrogen sulfide it is important to not to breath in the fumes as it can make you very ill and possibly kill you.
Cold as ice.
Regardless of the ambient temperature outside the lake it remains a constant 48.74 °F (9.3 °C). This place is almost ice-cold. With a frigid environment swimming is not suggested. Hypothermia would claim you in minutes. Special diving suits are required to keep divers safe.
It is to deep.
Even with proper diving gear the depths of this lake are hazardous. An experienced Israeli diver perished while on a dive to 394 feet. It took more than a day with special equipment to raise his body after it was found a week later. Another diver suffered from the bends after descending to 685 feet. He survived after spending more than a week decompressing in a pressure chamber.
Ancient treasure in the deep.
Interest in diving here is raised by rumors of ancient artifacts that may be preserved in the lake. A trade caravan passed by this body of water centuries prior. Others claim that World War II vehicles and munitions were dumped in the lake. There is also the possibility of locating the underwater cave that feeds this beautiful blue lake. If found it would make it one of the largest cave systems in the world.
Sources:
http://todiscoverrussia.com/blue-lake-in-russia-kabardino-balkaria-tserik-kel/
http://scribol.com/environment/lakes-and-rivers/the-mysteries-and-tragedy-of-russias-bottomless-karst-lake/