Ethereal Void

The Ethereal Void is a place within the fiction of UO.

It seems to be similar to the "Astral Plane" seen in Dungeons and Dragons, in that it is similar to space, but has breathable air.

It has been seen in the single-player Ultima games, but was not been seen in UO until the Mad Mage event, when the Blackrock Detector created a mirror image of Moonglow in it, within which monsters were found, and from which the Shadowlords escaped into Britannia.

Despite their physical similarities, there is simply no indication that the Ethereal Void is the same as the Sea of Stars that surrounds the continents of Malas.

For (slightly) more information, see here:

http://ultima.wikia.com/wiki/Ethereal_Void

Lord British is said to have taken a powerful artifact to the Void to guard it against Minax.