NOTE: I am really drunk please no ban <3THE TRUTH BEHIND UNDERTALE: THE STORY 'THEY' DON'T WANT YOU TO HEAR!!!1ONE
What letter does the word monster begin with? M. What letter does... Mexicans, begin with? Also M. Coincidence? Let's dig deeper.
What keeps Monsters in the Underground? The Barrier. What is a barrier?
'a fence or other obstacle that prevents movement or access.'So, a wall? Perhaps, Donald Trump's promised wall?
Undertale is truly the story of the election of President-Fuhrer for life Donald Trump. Believing that Monsters and Mexicans were the same thing, he banished them behind a giant 'barrier' and left them to rot for all of time.
But you may ask, what about things like spiders? They aren't really monsters! Well you are right, the spiders (aside from Muffet) are not technically monsters. However the spiders want money. What other culture has a (wrong) stereotype of liking money? Semites. Thus, they too were cast into the underground.
PROVE ME WRONG!
Protip: You can't.