It works as stands if "the missing continent" is a singular concept (which is the premise of the story, that this undiscovered continent was believed to exist).

It works as stands if "the missing continent" is a singular concept (which is the premise of the story, that this undiscovered continent was believed to exist). Compare "The tenth planet hasn't been found yet" vs "The planet that hasn't been found yet". If "the missing continent" was used then, like "the tenth planet" is used now, it works.