At the start of the game, go to the front of the mansion, then walk around the left side. Walk around near the fence there, and eventually Carnby will mysteriously start rising into the air. The screen will go blank and you'll end up in the underground passage where you find the dead body of his partner. The only way out of this is to reset, because you shouldn't be here yet and without the necessary items from the hedge maze, you can't finish the game.
Taken with permission from Flying Omelette.