Could be to a disconnected septic system. Possibly a dry well, old broken
line. If you break out the old plug with a cold chisle be ready with a
rubber plug in case it flows back in. I would trace it down some how, find
out if the house had septic and now sewer, ask neighbors that have been
around for a while if they might rember any changes to the sewer/septic
system be carefull I would think that it would havev been capped off if it
was intended to be used in the future.