Could not get good enough object definition on Google Earth or Bing Aerial Maps. Both images did indicate, as doe the last picture that you posted, that your yard is bowl shaped. As such it will serve as a catch basin for rain. Somewhere in the distant past it was probably a natural Pond that has filled in over the years. The excavation photo shows what appears to be fairly rich soil, which is expected in an old wetlands site. You are also quite close to the St. Lawrence River. What is you property elevation relative to the river level?