I just have a hard time imagining sitting on one.
I suspect, mostly due to your imagination, rather that the actual strength. I've worked around CNG, LNG, LPG, and lots of refined products and Petrochemicals. LPG, propane, scares me the most. The CNG, LNG are harder to ignite and outdoors quickly vent up and away. Large propane leaks will hug the ground and pool up to a large volume after a spill.