What can I do with a buried copper gas line?
In our neighborhood of 1970 built tract houses some homeowners added copper lines to a grill in back and a gas light front. The copper lines are invariably  connected to a saddle valve on the black iron gas service line above ground just before it enters the structure. (Fortunately no previous owner of our house added this.)

Some years back I helped a neighbor whose grill was non functional, dilapidated and whose gas light rusted out at the bottom and fell over. I made sure the saddle valve was off at the supply and I removed both the grill and the gas light. I cut off the copper lines below ground and flattened the copper tubing for 6" or so to seal it, then buried it. So there is 100 ft or more of abandoned copper tubing in the ground still connected to the gas service line.