[meteorite-list] What is this structure?

[Aleksandr Leonenko]

Hello!
I would like to know: what is shown in the photo - sinkhole or crater? A diameter of 15 meters. About 0.5 meters deep sand bars, followed by a hard green clay. Around the structure - a shaft of up to 60 centimeters. Unfortunately we do not have metal detectors, check for debris we can not.
What is this structure?

http://imageup.ru/s1085718
http://imageup.ru/s1085719

Sincerely.
Alexander.