Mercury

Mercury is a planet within the Terran Solar System, located at coordinates 5,4,5. In Ultima II it is possible to visit Mercury using a rocket ship which can be acquired in the year 2112 A.D., from the post-apocalyptic Soviet settlement of Pirates Harbour.

Description
Unlike the planet of modern scientific consensus, the Mercury of Minax's 2112 was largely covered with water, bearing only a scant handful of swampy landmasses on its surface. While no evidence of current civilization could be found on this desolate planet, a single dungeon on its major island suggested that intelligent life had once either inhabited or visited Mercury.

After Minax's defeat and the restoration of the time stream, it is not known if the planet returned to the arid world it has been found to be by today's observers - vacillating between burning extremes and profound cold and possessing but scant water in the ice of its deepest craters. It seems probable that it would return to such a state, however, given the lack of evidence that the astronomers of the Avatar's Earth recognized an oceanic Mercury.