In ten years to come an unknown enemy will rise up to come and capture the city and to take all of its technology for themselves. When the ancients got word of this they began their biggest plan ever, they had already been thinking about doing this project for some time now but, will now have to do it sooner than expected. This plan was called Escape from existence. In this plan they would have to dig and dig and dig and dig. This is how the Underground City came to be. It is still however unknown what it was used for. For all we know there could be a city down there, it may be just a tunnel to get out somewhere away from the city. The only thing that is known is that there are a ton of floors to the Underground City, it is not known how many or how big it is or if there is even a bottom floor to it.
It isn’t even known if the ancients are still alive, but it is known that there were no signs of a war or battle in the city at all, so it is assumed that there was no battle.