The simple answer is that with proper names ending in s both the default kind ('s) and the bare form (' ) are found, though the bare genitive is most widely used with classical, religious and literary names:
Bare genitive, s: Socrates', Xerxes', Moses', Jesus', Burns', Jones', James'. 's type genitive: Socrates's Xerxes's Moses's Jesus's Burns's Jones's James's
In those examples, the bare genitive is more common, with the 's kind as a variant. The same applies to your example: either the more common Columbus' or the variant Columbus's is correct.