We built the Arcologies to last for millennia, and they did; Through scarcity, devastation, war, and the loss of everything else. They housed the last remnants of humanity as the world held it's breath for a recovery that surely must eventually come..
This is part of the Great Lakes Metroplex, a lost city that stretches from old Chicago to the Milwaukee Ark. To the south, the greater Chicago area lies uninhabitable after the Little Wars, leaving it's northern suburb as the only shelter of humanity for a thousand miles.