Maretania is a far-off location mentioned several times but never shown. It's primary relevance is that a seemingly omniscient being named All Eyes lives in the caverns beneath it.
The caverns beneath Maretania are the resting place of All Eyes, a mysterious being of unspecified origin. He imparts information on those that reach the bottom of the cave system. Only three ponies are known to have successfully made the journey, Starswirl the Bearded, Lyra Heartstrings, and an unnamed zebra shaman who told Starswirl about the being.