To key to unlocking Pikmin 2's Challenge Mode(for one or two players) lies in the game's third zone -- the lake area. Once you have unlocked it, grab a group of yellow and red Pikmin as well as a handful of white Pikmin (to dig up hidden treasures along the way). No blue Pikmin needed. Fight the electric bugs and fire
emitters with the matching Pikmin and make your way to basement level 5. Fight the mechanical spider with a small group of yellow Pikmin: throw them at the spider's body, then call them back to you and move them out of the way when the spider is stomping its feet. When you have beaten the spider, a key (literally) appears. Have the Pikmin bring it back and exit the level by jumping into the geyser. The Challenge Mode will now be available for selection from the main menu.