It wasn't opening, but the first Titan in Unreal was so well done, first you see the Dark Arena from distance, then get to travel to it, enter and explore it, reading cool Nali messages about some frightening beast, and finally you get to the arena itself and walking to its center with mysterious construction and dead bodies - atmosphere that you could cut with a knife, adrenaline so high, you knew shit was going to hit the fan any second now, and when you finally overcame the fear and turned the switch, the ground started trembling and frightening sounds coming, the doors opened and in one of them the titan appeared, and you were so frightened, almost immobile from fear.
On my first playthrough I missed the prison cells under the Arena. Also I missed the Harobed village, and later turned out that the village had a semi secret graveyard behind it, and secret cave system under it. The game was like epic adventure, traveling alien planet from one place to another.
Deus Ex (2000) - a game that pushes the boundaries of what the video game medium is capable of to a degree unmatched to this very day.
