xkcd #3298: Aerospace Flowchart
Title text:
Someday, we will find the problem that goes with this solution.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3298/
I've heard of methane used to inflate gas bags to lift sunken boats but I haven't seen it used as an aviation lifting gas.