The key to success in most run-and-jump platform games is memorizing the terrain. You replay each level until you know how to time the moving platforms, know where the mini-boss will appear, etc. In Spelunky, though, the fun comes from not knowing. The worlds in this 2D platformer are "procedurally generated"—a buzzwordy way of saying that Spelunky makes everything up as it goes along. Each time your Indiana Jones-esque hero enters a new stage, the computer creates it on the fly. The upshot is that you never see the same level twice.
This programming trick energizes the game with a sense of adventure. Spelunky is tough—the manual advises, "Don't be afraid to die!"—but since you're always discovering something new, it isn't especially frustrating. Weird surprises, like a teleportation device hidden in the rubble, pop up just often enough to keep the game feeling fresh. Even after you splay your guts on a patch of insta-death spikes, ruining a long session of spelunking, the prospect of a new discovery is ready to entice you back for more.
None of Spelunky's conceptual cleverness would matter if the action were dull. Luckily, developer Derek Yu distilled his intuition for platformer rhythm into the algorithms that make Spelunky tick. The game maintains a nice balance of tight precision puzzles and breezier areas that let your caver stretch his legs. And while the visuals are chunky, everything in the game, from flaming frogs to yetis, moves with a carefully calibrated grace.
Beyond the game: If you like the procedural-generation idea, try NetHack, the classic RPG that has explored the concept for two decades. It's available free online, for every computer platform imaginable.
Worth playing for: Tunnel Man mercifully digs shortcuts to the upper levels of the game, making them more accessible to mere mortals. (You'll still have to work for the shortcuts, though. There's no free lunch in Spelunky.)
Frustration sets in when: You attempt to use the default keyboard controls. A gamepad is the way to go here.
Final judgment: Combine the intoxicating flow with a catchy soundtrack and an undercurrent of dark humor, and you get a game that will devour many lazy afternoons.