Pikipedia:Featured article

SUBMERGED CASTLE The Submerged Castle can be arguably one of the most agitating or hardest caves found in Pikmin 2. You are only able to bring Blue Pikmin with you, even if you figure out you can use Blue Pikmin's water patroling ability to carry non-blues onto land where you can get them under your command and enter the cave; this will never work. It has been programmed into the game, whether somehow you got them under your command or not, that they cannot enter the cave whatsoever. All hazards are found here, making it imperative that you command your Blue Pikmin carefully, though you can get Bulbmin. The worse is yet to come however, for the boss appears on every level, making it crucial that you devise movements to secure the cave's sublevels quickly. To get to the cave, you need to defeat many Yellow Wollywogs and tear down the black gate near Glutton's Kitchen. After the gate is gone, you need to defeat two Hermit Crawmads, Water Dumples and a Withering Blowhog. The cave's entrance is surrounded by undrainable water, which is exactly why non-blue Pikmin can't enter, it was programmed so that the situation would make sense.