Serpent Pillars

The Serpent Pillars are a huge teleportation gate in Britannia, leading to the Serpent Isle.

Description
The pillars stand in the arctic oceans of Britannia, surrounded by the ice of eternal winter, with a waterway ending just between them, which is mysteriously kept ice-free. The Serpent Pillars are a huge teleportation gateway to the Serpent Isle, although it is unknown how they came to be in the first place. When a ship goes between them, it is swallowed by a glowing, white portal, and transported to the Serpent Isle. The transporting process is hinted to involve the vessels flying during transit. 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 travelling back to Britannia exist on the Serpent Isle, but no one could find them, because the circumstances of their appearance are unknown.

History
Erstam and the other unsatisfied left Britannia through the Serpent Pillars prior to Ultima IV, settling the land after transit. It is unknown how Erstam got the information about the pillars in the first places, as he kept silent about it. Erstam spoke of attempts to find the matching pair of pillars for return to Britannia, but due to lacking information no one had been able to find them, thus in the end all attempts were given up. Gwenno also used them when travelling with the Fellowship expedition to leave Britannia prior to Ultima VII and it was never exactly cleared how she got the detailled information about the circustances of the pillars' becoming visible. In the beginning of Ultima VII Part Two, the Avatar and company used the pillars as well to travel to the Serpent Isle.

Though nothing was mentioned, the return of the compainions to Britannia prior to Ultima IX proved that somehow they must have been able to learn where to find the matching pair on the Serpent Isle.

Trivia

 * In the Original Serpent Isle Intro, the Serpent Pillars rise 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 game disks. The water wasn't originally frozen either; it was changed to cut down the disk space used by so much animated water.
 * 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 pairs, and are activated by saying a specific magical word in front of them. Graphically, the snake is slimmer and more tightly coiled around the pillar. One of them can be seen in the gallery.
 * No explanation is given how Erstam learned of the pillars in the first place, nor where they originally come from.
 * Interestingly, the loading screen of Ultima VII Part Two shows what transit through the pillars looks like from the perspective of the traveller. It can be seen in the gallery.