One day Papyrus got curious and went down into the basement while carrying a tray of spaghetti hoping to use it as an excuse, just in case his brother was down there.

He wasn't.

This let him explore the basement without worries but this in turn made him careless as he tripped over wires, hitting a machine by accident. The machine whirred to life and cracks appeared in space causing the disoriented skeleton to be sucked in and end up in an entirely different world unlike his own.

A world with an ancient civilization that was lost but resurfaced when a group of unlikely friends searched for its existence.

The world of "Atlantis".

Undertale and Atlantis: The Lost Empire crossover.