Lots of older homes have sewege cleanout access pipes in their front yards. These were installed by the plumber who had to trench the front yard to redo the sewer pipe going from the house to the city pipes. When they do the trenching to replace the pipe, they usually install two vertical access pipes so the future plumbers can more easily snake the pipes. For whatever reason all of the homes in my neighborhood have had this process done in the past few years.
If you have one of these front yard sewer access pipes, pop the lid and dump away.