There is an unused, cement or cinder block septic system under our yard that dates back to 1947 when the house was built. When we bought our home in August of 2013, the back yard was level. Now that we've been through a few freeze/thaw cycles, the yard above or near the system is indented in several areas by around six or so inches.

What type of professional should be called in to investigate and remediate the situation to ensure that our yard is safe from collapse?

Our home is a twin, and our attached neighbor has the same system in her yard. Her yard is showing no signs of change.

As an aside, our attached neighbor's father built the twin home using the frame from an old barn that he moved from a few streets over. It's a neat old house.