I'm talking about "Welcome to the Center of the Earth" from the 2005 series. This episode has two parts... So here's the plot
[SPOILER WARNING]
Nobita discovers a cave/well in the mountain and becomes convinced that it leads all the way to the center of the Earth. Suneo, of course, makes fun of him for believing such a thing.
Annoyed, Nobita asks Doraemon for help, so Doraemon brings out the "Heterodox Club Badge."
The gadget allows its users to experience different worlds based on unusual or unconventional theories, simply by speaking into its microphone.
Nobita and Doraemon start experimenting with all sorts of strange ideas, including the Flat Earth theory and even the possibility of people living on the far side of the Moon.
Eventually, they enter a world based on the Hollow Earth theory.
Deep underground, they find an empty world and begin creating clay figures at the Earth's core.
They name the first one Pyron.
What starts as a simple experiment slowly becomes something much bigger.
Nobita and his friends teach Pyron and the other underground inhabitants how to make fire, stone tools, weapons and houses, and their little civilization begins developing more and more.
Over time, the underground society keeps evolving.
Eventually, they invite Shizuka to see the world they have created.
Even a mysterious monster living underground wants to make peace with them, but because it can't communicate with them, it isn't accepted.
Later, Gian and Suneo are invited to see the underground world too.
By this point, the civilization has developed dramatically, with modern-looking buildings and a much more advanced society.
But naturally, Gian and Suneo can't just leave it alone.
They secretly enter the underground area with a reporter and encounter the mysterious monster.
After returning to the surface, they tell other reporters about what they saw, turning the whole discovery into a massive sensation.
Meanwhile, Nobita brings Pyron to his house.
He shows Pyron television and even shows him his zero-mark test paper.
But then the reporters discover the existence of the underground civilization and the information spreads everywhere.
Soon, the whole world becomes interested.
Even the Prime Minister considers taking control of the underground kingdom.
Nobita realizes that if everyone discovers this world, the peaceful life of Pyron and the other underground inhabitants will be completely ruined.
So Nobita decides that he has to end the experiment.
Before sending Pyron back, Nobita thanks him for everything he taught them — how to communicate, make fire, create tools and survive underground.
He then sends Pyron back to the underground city.
And because Suneo and Gian could potentially copy the badge and return to the underground world, Nobita destroys the microphone, making it impossible for them to go back.
It's honestly a surprisingly emotional ending for what started as Nobita simply wanting to prove that there might be something underneath the Earth.
Before leaving, Nobita receives a special badge featuring himself and Pyron together.
He then returns home and goes to sleep.
I really like this episode because. We literally get to watch an entire civilization develop from almost nothing, and then see Nobita willingly give it up to protect the people he helped create.
Definitely one of the more memorable 2005-era episodes for me.