In the first years of my professional career, I was working in ground investigation project for a superstructure. This engineering structure is located in a karstic region and affected by the differential settlement. So we were investigating for any karstic activity below the structure.
We had a site visit to adjacent karstic features to understand the karstic mechanism of the region, by this way we could compare the settlement with other depression mechanisms.
I was interested in one particular location amongst others. That was an excellent example of collapse sinkhole, but there was something wrong.
The radius of depression of the sinkhole is about 40m and area was swampy. It was apparent that area is seasonally becoming a small lake and with the withdrawal of groundwater, it became a swamp.
The confusing thing was tens of dead fish were in the swamp. I have not found any reasonable reason to explain that dead fish are there. Because it is a seasonally active lake, there is no river connecting to the sinkhole, and there is no lake in the close vicinity of the lake. Then I forgot to examine it deeply due to workload.
“This is the story provide my enlightenment.”
After couple years later I turned back to the same region for another ground investigation works. While I had a conversation with locals, a guy told me a story about a lake which is close to my mysterious collapse sinkhole. This was the story provide my enlightenment.
“This is our village’s magic lake. Occasionally this lake includes water, at all other times, it is dry. Years ago, the water level was very high in the lake, and we had a wet season in the area. Toward the end of summer, the water level started to decrease, and suddenly lots of fish appeared in the lake. With decreasing the water level, fish are not able to fit into the lake and oxygen level was very low for that much fish. All the people run to the lake to collect the fish, and all of them thanked God because they thought God sends that fish. This event heard by everyone around the village and city. Later the truth is understood. There is a big lake about 10km away (as the crow flies), with increasing of the water level, fish are travel to this small lake inside the karstic connections. When water level started to decrease fish have stayed in the small lake.”
When I heard the story, I remembered my site visit and understood why dead fish were there. I never thought that fish could come from 10km away inside the karstic rocks and I realised that underground karstic cavities are fabulously long and big in the region.
Still, I can hardly believe that karstic cavities are continues at least 10km.