Horuulk

Horuulk was a planet in the former Romulan Neutral Zone, once home to (or controlled by) an alien race believed to be extinct. The planet featured a monumental work of engineering created by this race, eons before the rise of the Federation, in the form of metallic bands which circled the planet. Although their purpose was never made clear, they were deemed mundane in basic function, yet likened to Earth's Egyptian pyramids in historical significance.

As late as the 24th century, Horuulk lacked an ozone, exposing its surface to lethal levels of ultraviolet radiation, making it uninhabitable. It may be for this reason that the Romulan Star Empire never expressed territorial interest in the planet.

In 2386 Stephen April experienced a holodeck facsimile of Horuulk's surface. (Star Trek: Arcadia)