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.

Trivia

 * The original introduction of Serpent Isle pictures the Serpent Pillars rising from the depth of the Ocean before engulfing the Avatar's ship, whereas the final game shows them basically just standing there in the Polar Regions of Britannia. The original depiction is actually much more consistent with Erstam's description in Beyond the Serpent Pillars but had to be cut due a lack of space on the disks.
 * Ultima Online actually has Serpent Pillars as well, serving as a gateway between Britannia and the Lost Lands. Unlike the ones in the core series however, they don't go in pair and are activated by saying a specific magical word in front of them.