This secret achievement, entitled "Conclave," will be awarded at the conclusion of the 4th campaign mission of the Modern Times DLC -- if you're progressing through the 12 new campaign missions, this is an unmissable achievement.
A tip for this particular campaign mission: Have enough Construction Offices to be able to re-build the myriad of buildings that will collapse over the course of the mission, and save your progress whenever you see the "Disaster Imminent" notification, so you may try to minimize the damage of so many earthquakes/volcano eruptions (though, the earthquakes are the bigger threat, of the two, during this mission).