I live in an area where 99% of homes have gas lines, but mine does not. I called 811 and the gas company showed up, but said since I have no gas service, no pipes run under my property. When the electrical company showed up, they said it is very unlikely there are no gas pipes, and seemed suspicious the gas company was diligent in marking the area. I'll only be using a hand shovel to dig. Are gas pipes made of a material that can be broken with a hand shovel?
I speak as a former engineer for a utility that provides both gas and electricity.
Gas lines are typically metal or plastic. If plastic, they're very likely thick enough to withstand most hand digging with a shovel. Laws typically require hand digging within some distance (18" in my state) of a marked line. If you do find a gas line with your shovel and notice any damage beyond, perhaps, very minor scraping, call the gas company back. They'll evaluate and will almost certainly be responsible for any costs of repairs since there were no markings.
It is also possible that there are underground lines that are not owned by a utility company and will not be marked.
Just dig reasonably. Age, manufacturing defects, and other factors can create all sorts of variations in the strengths of the lines, whether plastic or metal. Hand digging, though, is unlikely to create a spark that would ignite the gas, even in the very unlikely event your shovel does penetrate the line (still evacuate, call 911 AND the gas company, and get your neighbors away, too). Just don't smoke while you're digging!
Finally, while there may be warning tape or other devices, there are no guarantees! These things are installed by humans who don't always do everything perfectly, nor were they used in the oldest installations. Easements are a good indication; just be aware that an easement on a map may not reflect the actual location of a line, and a "blanket" easement gives NO indication of where the line might be.
Please also know that utility lines are almost always buried at proper depths; however, there is no guarantee that someone did not change the grading of the property after a line was installed. When people would ask me how deep our lines are buried, I'd answer "all the way to the bottom of the trench".
Not something to take less than seriously (the hose aimed upward keeps the fireball from curling around and burning the firefighters from above and behind). The fire continues until the gas is shut off, despite the water.
The tank in the photo is probably the most common size for residential propane customers. Buried lines are likely big enough to eliminate any claims of a "small" fire... and how much damage will be done, even if the shutoff is not engulfed, in the time it would take to shut it off? Sure, there are always rare exceptions. Do you care to place a financial wager with me that has a 99% probability of your losing? Stakes are even higher here.
Just an experienced digger answer here... I worked for landscaping companies in my teens and while some of the lawns were marked, many were not.
Basically you want use a tipped shovel - not a straight edge and dig down around 45 degrees and about 10 inches down at a time, one shovel full. Not fool-proof as anything can happen but if you do this you are 99.5% of the time either not going to hit it or glance off of it.
Also if there are trees nearby you want to have an axe handy. The times when I have seen electrical/gas lines hit by hand digging were when guys were getting too aggressive with the pressure due to tree roots and/or rocks.
And to answer your question - an open gas line is not safe but generally not dangerous. An open gas line in a house over time... yes. Obviously you don't want sparks or fire but the dilution of the gas in the air makes it very improbable that there would be an explosion. I have had to quit working at least 10 times on a house waiting for gas company to come out for a repair. It happens.
A gas line is usually either iron or steel (older installations) or HDPE (i.e. plastic used in newer installations) and either one can be damaged by careless excavation. Often the steel/iron lines have been buried for a long time and have been weakened by corrosion both internally and externally.
HDPE does not corrode but is fairly soft and can be cut by a shovel.
If there is gas on your property, there should be a shutoff valve somewhere between the service line, often in or near the street, and the home. But if there is no current gas service in the home, no meter outside, and the gas company says no lines on the property, there is little more you can do.
If you suspect one is there anyway, the best type of excavation to use is with a vacuum machine. You may be able to rent a smaller model or you could hire a local vacuum excavator service.
Puncturing a gas line can be dangerous as the gas can create a fire or explosion hazard. That can really mess up your project!
Both are correct, you have no service pipe to your house , there is a distribution pipe near the street, on your side or the other side. This distribution pipe is deeper than you are digging and (as answered) is steel (not iron) and stronger than your shovel. It is very likely cathodically protected so has had no corrosion . You do have deeply buried water and sewer lines. If you have a buried telephone line you are likely to hit and damage it; I have damaged them twice in different houses. Sprinkler water lines are also targets.