Removing pole and concrete base from yard
This is the remnants of a natural gas or propane grill, gas heater, or perhaps a gas lamp. The flared tubing fitting and old control valve stem are the giveaways here. 

Look around the structures adjacent to the yard for signs of a pipe with a valve for which you cannot ascertain a purpose, so you can at least try to confirm that any leftover fuel supply line is shut off. Although I doubt anyone would be callous enough to cap, bury, and abandon a live gas line, it is not out of the realm of possibilities. There is also the chance that the fuel supply came from a portable tank...

Either way, I would start digging until it was excavated, then chuck it. If in the course of digging you encounter an old pipe or buried tubing, be careful and try to locate the source to ensure it is shut off and safe to leave as-is.