Suzanne Martel’s 1963 The City Under Ground is the Anglophone version of Quatre Montréalais en l’an 3000, also published as Surréal 3000.
The underground city of Surréal1 was founded by survivors of the Great Destruction, survivors who had escaped the disaster by taking refuge in a geothermally powered facility under Montreal’s Mount Royal. As far as the founders knew they were the only humans left on Earth. By the year 3000, no visitor from outside Surréal has appeared to contradict this belief. Indeed, so convinced are the inhabitants that the surface of Earth is still uninhabitable that nobody has checked conditions outside for centuries.
A small earthquake changes everything.