Underground as in programming for fun or as art, because you enjoy it as a hobby/subculture. Opposed to the "above ground" programming world of programming for

Underground as in programming for fun or as art, because you enjoy it as a hobby/subculture. Opposed to the "above ground" programming world of programming for other people (possibly also for money, but OSS is often just for recognition) where people have demands about unimportant features, terminology, coding styles, codes of conduct, etc. Sometimes they get quite aggressive about their demands, too.