The Mystery of Deltarune's Old Song from the Sea

Deltarune’s old song from the sea seems like a one-off mention of something vaguely weird by Onion, an already strange character. However, veterans may know that the history of the old song goes all the way back to one of the strangest mysteries from Undertale, still unsolved to this day. With a bigger picture and a lack of unified information, I decided to make this video. Join me for a dive beneath the waves.
Timestamps:
The Old Song From the Sea (Introduction): 0:00
Tra la la… (Undertale’s Old Song): 2:47
Do you wanna… y’hear a secret…? (Deltarune’s Old Song): 9:07
Behind That Rushing Water… (Why is the Old Song?): 14:23
Quiet Water (What is the Old Song?): 21:21
Resting Her Voice (Outroduction): 27:37
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Music used:
Welcome to the World of Pokémon! (Pokémon Mystery Dungeon: Explorers of Sky)
Waterfall (Undertale)
My Castle Town (Deltarune)
Here We Are (Undertale)
findher.ogg (Deltarune?)
The Dark Truth (Deltarune)
Quiet Water (Undertale)
ANOTHER HIM (Deltarune)
It’s Raining Somewhere Else (Undertale)
Undella Bay (Pokémon Black 2 and White 2)