For the Geo Quiz, we are looking for a vast area that used a common currency, about 2,000 years ago.
It wasn't just any area, it was an empire and it didn't exactly invite new members, but conquered and annexed them.
This "single currency area" was headquartered in one of the greatest cities of ancient times and its seven hills straddled the Tiber river,
Its emperor ruled with absolute power, but he did let far-flung provinces have some autonomy.
If they had their own currencies when he took them over, they could keep using them, along with the empire's single currency.
The answer is Rome, which used a single currency across its vast empire for 400 years.
Anchor Lisa Mullins talks with former banker and now historian Gilles Bransbourg, a researcher at New York University and the American Numismatic Society about how the Roman Empire's use of a single currency differed from the eurozone and whether there are any lessons to be learned from the ancient period.