Did you know that there’s a (formerly) secret “closed” city in the Gobi Desert called 404?
Its full name is No. 404 Factory of China National Nuclear Corporation, but it was more generally known simply as “nuclear town”.
To this day, 404 appears on no maps. Search where you will (aside from the actual Gobi) and it will be a case of “404 City Not Found”.
For more interesting closed, lost or hidden cities, check out Secret Cities: The Haunted Beauty by Julian Beecroft.