This is the Underground. It’s a place where Undertale based characters are typically sent.
The parts of the cave are
Which is below and is one of the easiest ways to access the Cave System.
Which refers to one a small subset of caves with abnormal properties, inhabited by a group of lost children and the loving lady that takes care of them.
Which refers to the most clearly Undertale based area, which also is what caused this Doc to be written in dark mode, Home to cool areas, the T.M.M. Labs which is one of my oldest things, and more!
a cursed place where the monsters can never truly die, the caves hold a place for them.