Just so you know, Eternal Darkness for GameCube is one of the greatest games of ALL TIME. The developers worked so long and hard on it(about 4 years) that nothing was overlooked. If you ever get a chance to play it, you must. Don't look for it on any other systems though, it is a GameCube exclusive title. The games sanity effects are so in depth, they'll make you question your own sanity, let alone the sanity of the character you're controlling.

how can they work on it for 4 years when gamecube has only been out for like a year?

It was originally in development for the N64, but a lot of the things they wanted to do with the game weren't possible on the N64, so they switched over to developing it for the GameCube. Since the developer, Silicon Knights, has a very close relationship with Nintendo(they only make games for Nintendo a.k.a First-Party developer), they were able to do this because they had access to some of the technology and stuff. Because of the amount of time they worked on it, the story is amazing, the graphics are amazing, they are no bugs(except for the very, very rare occurance of an object clipping through another), the gameplay is amazing, everything is amazing. The sanity effects help to emmerse you even more, and are really cool.