The trick behind successfully producing a television series lies in making the most out of the money available. In a science fiction series the budget is even tighter - once money is allocated to special effects and model building and other shiny things and once the primary sets are built, the money for new locations is extremely limited.
Planet Hell is the Star Trek world's answer to this problem. It consisted of a large, filthy-looking pit with a variety of levels and passageways, as well as a better-than-average lighting system rigged above it. By altering the way the pit was lit (as well as by employing the tried and true Star Trek technique of shaking the camera a lot and having the actors fall in dramatic ways) the production crew could make it appear to be anything from an idyllic cave to a earthquake-prone, thunder and lightning plagued hell hole. It was mostly used for the latter (who wants to watch a sci-fi picnic?) and was nicknamed Planet Hell by the cast and crew. The name stuck.
The pit was constructed early in the run of Star Trek: The Next Generation (remember Galorndon Core? Actually, don't answer that) but was used extensively in Deep Space Nine, Voyager and (I'm assuming, though I'm not positive) Enterprise. The set is about as close to the environment of an alien world as one can get without actually filming on location, something that was rarely done on the Next Generation; DS9 and Voyager left the sound stage far more frequently.
The downside to this is that viewers begin to believe that most planets exist only as a series of interconnected caves that are prone to strange rockslides, energy disturbances and other tension-inducing plot devices, but I guess Star Trek isn't exactly the pinnacle of television realism.
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Servo sayeth: The Hell set was expanded for Star Trek: Insurrection and the various Trek TV shows benefited from this expansion, too. Voyager actually called one of the planets they visited once "Planet Hell" as an in-joke to the set. The commercial was stupid though. Something like "To find life-giving energy, Neelix must go to Hell!"