Abandoned Ecumenopolis

The disappearance of this planet's original population remains a mystery. Once the shield went up and they were isolated from the rest of their interstellar empire, they likely faced mass famine and civil unrest as they struggled to support their capital's massive population with what limited resources they had.

Yet there are no signs that they demolished parts of their city planet to cultivate the land. Nothing suggests that any significant fighting took place after the shield went up, and there are no remains, fossilized or otherwise, of the inhabitants. It's like they simply vanished into thin air.

The eerie and empty streets of this abandoned planet-wide city still do not betray what happened to the original inhabitants. Posters and pamphlets have been discovered that mention some sort of "Evacuation Day." They are typically accompanied by pictures of a galaxy that our astronomers do not recognize:

"EVACUATION DAY - TODAY IS THE DAY. REPORT TO YOUR NEAREST RELOCATION CENTER AS SOON AS POSSIBLE TO BEGIN YOUR NEW LIFE."

The planet was clearly abandoned after this "Evacuation Day" concluded, but how? There is no evidence that the planet's energy barrier ever went down.

Clues have been discovered that indicate the pursuit of some sort of massive research project that predates the shield. Resources and science staff were diverted to this project in increasing numbers, and efforts appear to have greatly intensified after the shield went up.

After intense search efforts, we have discovered where the inhabitants of … went. Deep inside a tall fortified structure near the planet's northern pole, we found a relic that our archaeologists are convinced holds an actual miniaturized galaxy.

Somehow, after creating this galaxy, the inhabitants of the Relic World all relocated there and then sealed it in this container with some kind of impervious subspace field. While we can't interfere with the galaxy itself, our scientists assure us that we can study it.