The Vienna forest was once a hunting ground for nobility. The forest in its southern region suprisingly has the largest underground lake in Europe.
 
According to legend, it was originally a limestone mine.  After years of excavation, the underground tunnels went deeper and deeper, and  extended in all directions. During a blasting operation, 20 million liters of water were released and formed the underground lake.
 
The mine was used to produce jet fighters during WWII.
 
While walking through the dark underground mine, we encountered some caves where only one person could pass while other caves could have hosted a whole wedding for hundreds of people.
 
I said, "What a nice place for a religious retreat!"
 
Others commented, "A cool quiet place."
 
The guide said, "In the early days when mining just started, horses were used to transport the limestone. These horses later became blind upon exiting the mine after working for so long in the darkness."
 
I added, "Dharma practitioners taking retreat in dark places must also come out similarly at the end. In leaving, they should gradually adapt to the light by first letting in little light, then more light, and eventually full light. If they fail to let in the light slowly, their eyes may also get damaged.”
 
Finally we arrived at the underground lake. The water was so clear that one could see the bottom.
 
I asked, "Why are there no fish?"
 
The guide answered, "Fish can't survive in limestone water."
 
We rode on a boat traversing one cave after another in the large cave lake. 
What a rare once in a lifetime pleasure it was to cruise and appreciate the scenary of the underground lake!  It was a wonderful sensation.  

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Transfixed and amazed by the majesty of the lake I inadvertently entered meditation.
 
The universe is composed of four elements: "Earth, Water, Fire, and Wind".

Earth is the ground.

Water is the ocean.

Fire is the underground lava.

Wind is the air.

Practitioners of Vajrayana Buddhism also believe that:

Earth represents "bone and flesh".

Water the bodily fluids.

Fire the temperature.

Wind the breath.

What would be the result if we combine the elements of Earth, Water, Fire, and Wind in the following ways: "Water and Earth", "Fire and Water", "Wind and Fire", "Earth and Wind"?
 
We should consider how a life begins and ends to  comprehand the original form of life.  Vajrayana Buddhism practitioners achieve enlightenment by unifying these primary elements.
 
Yet, I still think that there should be fish in the underground limestone lake!

I believe that where there is water there should be fish. There must be some kind of fish that can prosper in the limestone water given the universal truth that there's always a controlling factor for every object in the world; it is never absolute. Therefore, a fish will be discovered if an effort is made to study this.
 
So, dare I ask, marine biologists, do you know any fish that can survive in a limestone lake?