It's a gas, man
Lest anybody think that 200 bar gas cylinders are out of the ordinary, they are pretty much standard for any application that uses gases such as nitrogen. They're the 5ft high, 10 in wide ones in the corner. Or if you like, the one rocketing along the street in the stupid prank video you downloaded.
Oh and if you have a CO2 cylinder, there's this great trick for making dry ice......