The concept is unnerving enough. Trapped in a dark and forgotten metropolis, miles below the ocean's surface, you must battle a population of mutated, tormented souls to avoid becoming one yourself. Images and videos of this place - with their hulking mechanical monsters, corpse scavenging children and genetic sideshow freaks - only deepen the dread.

But nothing can prepare you for actually playing BioShock... for walking its murky halls and meeting its demented denizens personally. Nothing can prepare you for the experience of this submerged nightmare up close. The world of Rapture doesn't just look scary. It is scary.

Total immersion is the key. From the moment your plane crashes into black sea water at the beginning of the game, you are in complete control. Swimming to the nearby lighthouse and taking the mysterious elevator down into the city, although it is the only available path, feels like your own fateful choice.

Other major turning points are similar. They don't occur in loaded cut scenes, but in sudden and unexpected sections of gameplay. You're introduced to the Little Sisters and their penchant for harvesting bodies as you silently creep by one on the catwalks above her. The first Big Daddy heaves a screaming victim straight at you before stalking off, untouched, into the gloom. And you immediately understand the fragility of Rapture when a tunnel you're walking through is shattered and flooded by a sinking piece of your own plane's debris. Even the game's tutorial is handled seamlessly and invisibly through the voiceover of a mentor character, Atlas.