What do you think the most untapped source of worldbuilding inspiration?

I'm talking works of fiction, real-world cultures, even underutilized mediums. For me, it has to be travel guides; they offer unique snippets of history and culture but leave much unsaid, prompting you to fill in the blanks, not to mention that they get you thinking about things that are otherwise off of your own personal radar.

So, what do you think needs more love from all the writer's-blocked worldbuilders?