Once you do wander into the Underground Waterway, and you'll see a wheel not far from the entrance. Once it's struck, a nearby portcullis will rise. It's only the first of three grilles, and the last one is the toughest to pass.
Keywords: Kingdom Hearts: Birth by Sleep
Tags: grilles, kingdom, psp, guide, sleep, games, kingdom-hearts-birth-by-sleep, by, hearts, guide-walkthrough, walkthrough, birth