Board Thread:General Discussion/@comment-27524978-20160701164517/@comment-29683173-20160818225310

I would say that the Underground cannot exist in our world, that we know of. Let me explain:

First, it is never mentioned whether or not the Underground has a day/night cycle. It is explained that monsters have the ability to create artficial lights that can mimic the sun and stars, as seen in Waterfall (they might actually be crystals). There is a considerably large portion of Undertale that isn't directly talked about aside from maybe a few times by Toriel, in the Ruins or at the Snowdin library--magic. Multiple monsters seem to be able to use magic. The explaination for why humans can't use magic is unclear, but it can be assumed that it is because of the differing nature of their souls.

A solution for why Waterfal can exist so close to Hotland could be that they are farther appart than we are lead to believe. Frisk and the monster kid walk for a good while before reaching the area where you fight Undyne in nuetral and pasafist. Hotland could be a dormant valcano. This would also explain why the monsters were confadent enough as to build the Core there, however timeframes are a huge mystery.

My theories have a good few problems, but that is largely atributed to the lack of knolage relating to the Undertale universe.