Wherein the Underground is separated by the mess that was once Waterfall. You have First Home to Snowdin, where things are relatively vanilla and "nice" - then you have Hot Lands to New Home, where things are quite the opposite. As for Waterfall, it has become known as "The Meat Caverns" after the first Royal Scientist post-banishment created a fungus that grew out of control in the humid conditions of that section of the caverns. It is now a red, spore-filled mess that monsters try to avoid whenever necessary. If you need to pass through it, it is advisable to wear a mask. There is a tunnel dug out beneath the caverns, but it tends to be forgotten about regularly... And it's poorly lit, besides.