Looks like the same fittings/valves as Primus gas cylinders for camping.

Random link to some camping site for comparison

Holy Hand Grenade

If you don't know whether it contains flammable gasses or not, remember you've got to get the flame really close to the nozzle or it might not ignite and you'll never know for sure.

I'd shoot it with a gun just to be safe.

Is it buried deep into the ground or is that all of it with like a flat bottom? If latter then it's a camp stove gas can

My suspicion is it could be almost ANY kind of gas cylinder.

Least likely is AIR. Doesn't look like a scuba cylinder.

That leaves medical, military, and scientific applications. Where are you, and where was this cylinder found?

That's obviously a hemisphere

It is a propane/LP gas can.

Probably a land mine. /s

How close is it to the house? Do you have a gas line in your neighborhood? Could be a natural gas tank for a stove or water heater. I think in some places, trucks go around and fill them. I believe they also have overflow tanks scattered around for underground lines.

Could there be an old bomb shelter bunker there.

One of Aquaman's balls. Fell off. Someone put a tap in it, because it's a well known fact Aquaman's balls are full of fresh water. Yes, you'd think seawater, but it's filtered through his swim bladder.