system
Given the following post, respond as if you're commenting on Reddit:
human
[Mario] Who laid all those pipes and why?

People don't seem to realize how much pipe there is.  Remember, there are pipes on far distant planets, pipes you can use to cross the continents or even the galaxy, pipes in the clouds, pipes that lead to nowhere, and of course pipes that have no use.

And yet despite this, they all seem to follow a sort of standard.  Who went out of their way to lay a cross galaxy piping system, why, and most importantly *where did they go?*
gpt
The Mushroom Kingdom we see is only a shadow of its former glory. The Mushroom Kingdom was a human empire spanning entire worlds. Once they conquered almost everything, the humans started to dwindle. Nowadays there are only a handful left, and most of those are in the Royal Family. Perhaps ironically, one of the most easily-conquered and servile of races, the Toads, are rapidly becoming the de facto inheritors of what's left of the Human Empire. 

Remnants of the halcyon days of expansionism are everywhere. It hardly seems like you can walk more than ten miles without finding an old brick road or a crumbling fort. At its height, the Mushroom Kingdom developed a series of pipes to allow rapid transit, for both convenience and safety (as pipe-travel often circumvented dangerous wilderness). The pipe also served a militaristic purpose, allowing spies and shock troops to be quickly dispatched to enemy lands (including the territory of the Koopa Kingdom). The vast pipe infrastructure was developed over the Mushroom Kingdom's history, centuries and centuries of exploration, building, expansion, and further exploration. It took a long, long time to lay all that pipe. 

Those who would explore unknown lands and dig tunnels to lay the pipe were regarded as incredibly brave, as only a very select few could stand to 'plumb the depths' for the good of the Empire. Eventually they became a kind of class of knight-errants, often serving at the will of the Kingdom's ruler. Few plumbers exist today, but those that do are known as heroes and are still as venerated as they were in ancient times.