These are the remains of a limekiln. We found it on a manor map from 1799 in an archive, located the place by some landmarks (a pathway that had stayed in the same place for two centuries and a stream that had been relocated into a trench during Soviet time land improvement). The remains were completely submerged and invisible from the ground surface, but the area could be detected as a patch of different vegetation).
A picture was taken every minute, it is now played back as 25 fps video. Enjoy!
(Things to do better next time - set manual exposure and whitebalance to avoid the flickering)