Serpent Pillars

The Serpent Pillars stand in the Polar Regions of Britannia, surrounded by eternal winter, with a water way just ending between them. The Serpent Pillars are a huge teleportation gateway to the Serpent Isle, although it is unknown how they came to be (that they were created by the Three Serpents of Order, Chaos and Balance seems most likely).

When a ship goes between them, it is swallowed by a white portal, and transported to the Serpent Isle. The Pillars aren't always visible. They only appear in winter, when both Moons and the sun are at the same time on the sky. A matching pair for the travel back to Britannia exist on the Serpent Isle, but people were unable to find them, because they had no idea when they appear.

Erstam and the other unsatisfied left Britannia through the Serpent Pillars prior to Ultima IV, settling the land. Gwenno also used them on her expedition to leave Britannia prior to Ultima VII. And in the beginning of Ultima VII/2, the Avatar and his/her Companions use the pillars as well.