My mother’s yard had a house on it before 1900. The foundation of it laid in a square when I was a child. Her house was built about 100 ft. in front of it in about 1909. There is or was an old cistern about 4 ft beside her house. It was never noticable before, but this Spring it has raised up into a cushioned mound.
First of all, I am not sure exactly what a cistern is and secondly does anyone know what is happening?
A cistern is a container for storing rainwater. I guess that digging a well in that area would have been too expensive, and that reticulated water and galvanised iron tanks were not available at the time it was dug.
Perhaps your cistern (being empty and lighter than soil) is floating up through the soil. Have you had rainy weather recently? I would recommend that you have it filled in.