Natural gas pipelines can be buried as little as 24 inches to as much as 8 feet deep depending on the location.
Usually main gas lines are buried at least 4 feet or even deeper depending on where it's at an the terrain where it's buried.
The natural gas line to my house was buried at 4 feet deep but I've seen some natural gas pipelines buried only 24 inches to homes.
The natural gas pipe to my home is plastic so it can be very easy to dig into it if you're not sure where it's located.
So always call the number 811 before you dig in any area of your property to prevent digging into the natural gas pipe.