Planting tree - hit sever line?
Clay has been used for a long time for sewer pipes - and for transporting water in general (google "clay sewer pipe California").  It could also be a drain tile installation - like a french drain.  The simplest way to tell would be by having someone flush the toilet, or take a shower (something that will generate flow in the sewer) and look inside the pipe to see if there is more flow.  You could also  call 811 (Digsafe) or look up building records with your local town hall to see where the sewer line is on your property.