It grew unabated
Every month it could shelter
Another few million souls

We tested in on Ganymede
Within less than a year
The colony was thriving

But it did not stop growing
In the subsequent years
It covered the surface of the moon

It grew and grew
Until it deemed its very denizens
A threat to its survival

Few escaped and those that didn't
Were forced to hide
In the recesses of that endless city

In time the city engulfed
Not only the surface
But the depths as well

It grew until nothing was left
But a living maze
Of metal and machinery

A place where none
Now dare venture
The planet city of Birulon