As long as we are telling interesting stories about gas lines....
I was working on the Central Artery project (large highway project) in downtown Boston many years ago. We were doing the geotechnical analysis for the project, so we had to drill lots of holes. One day my field engineer calls me to report that the driller had drilled clean through a 24 inch diameter gas main, and it was leaking badly, and the job was shut down.
OK, so the gas company needs to fix the main. I assumed they would shut the line down and weld a plate over the line. Not so. Turns out that, as has been mentioned, natural gas is only flammable within a certain range of oxygen/gas mixture. There was so much gas that the area surrounding the line was not going to be flammable, at least that is what the gas company said. They excavated the line (it was about 6 or 7 feet down, sent a welder down into the hole with a scuba type tank, and welded the line shut without shutting it off. Neither I nor my field engineer wanted to be within a hundred miles of this operation, it sounded pretty crazy, but the gas company said to relax, they do this routinely.
Now I am NOT recommending this to any homeowner, DIY'er or anyone else, just saying......