The lore of Undertale has many secrets within.. And I think I found two of them.
The game begins with a human character (actually Chara) falling into Mt. Ebott, in the year 201X. We then skip who knows how long after, when Frisk, the player character also falls into the mountain.. But you see, the first clue is waking up in said mountain. Apparently, he was resting in the flowers for a while. How long? We do not know. But we do know that Frisk had just fell down SEVERAL miles (this is a mountain, after all.) But wait, how is someone supposed to SURVIVE such a fall? "Freddy, this is a fantasy game!" Yeah yeah, maybe it won't actually happen, who knows. Probably won't, we have no evidence of anyone SAVING. But that's not my only piece of evidence, as I also have the battles in the game. What is the ONE thing you battle with through the entire game? Your SOUL. I would pass this off as a choice of battle, but you battle the BODIES of the monsters, NOT thier soul. Why is this? Also, careful examination of the battle intro shows that Frisk actually BECOMES the soul.
https://m.youtube.com/watch?v=nCHwdf73NDc (0:20, watch Frisk BECOME the heart)
This is not true of the monsters, who dissapear, and then thier soul is shattered. What does this all mean? It means that Frisk actually DIED at the start of the game. The entire game of Undertale is thus, Frisk's afterlife. He uses his soul to battle beause that's all that's left, he IS his soul. As the writing in Waterfall says, a human SOUL can presist even after death.
But, another theory I have. You know about Hard Mode, of course. You get it by naming the first human (Chara) Frisk.. Frisk is the person you play as in the game. So, if you make it so Chara IS Frisk, then that means you're playing as the first human. Hard Mode is in fact, a PREQUEL, where you play as CHARA, the first human to fall. That explains the different monsters, as this was DURING 201X. (That also means Chara died too, which may explain why she was actually a demon many years later)