Although Peroxide has ceased to be, some of their old projects are still archived on their site and elsewhere. The Underworld Viewer is a simple utility that renders all the levels of both Ultima Underworld and Ultima Underworld 2 using whichever acceleration mode you prefer so that you can view them and wander through them. It may also be possible to load the level models into a 3D modelling program, if the images on Peroxide’s site are any indication.
And Telemachos, formerly of Peroxide, never seems to run out of ways to keep busy. Building off of the Underworld Viewer, he produced an actual editing kit, the Underworld Game Creation Kit, with which to edit different aspects of Ultima Underworld, as well as walk through all the levels and playtest them.
Both utilities require the original data files for the Underworld games in order to function.