Anodes beneath your feet
There are a lot of words just set into the ground if you know where to look. Most of them have to do with the invisible infrastructure which makes our modern world work. I always wondered about the ones labeled "anode". I'd spot them here and there but never saw one labeled "cathode". It seemed strange that there would be a bunch of one and none of the other.
I finally figured out what was going on after seeing one clearly labeled as belonging to PG&E. After doing a little research, it turns out the magic words here are "sacrificial galvanic anode". They're supposed to protect nearby buried metal structures from corrosion.
When the thing you want to protect is a natural gas line, it's obvious why you'd bother with something like this. After all, bad pipeline management leads to craziness like what happened in San Bruno in 2010.
Call it totally random, but somehow these bits of knowledge turn out to be useful now and then. Not everything is related to computers, after all!