Near the Weather Institute in Emerald, on route 119, I believe, there's that little river. If you surf in it, there's waterfall that goes up. You know where I'm talking about, hopefully. Well, if you go up that waterfall, there's some bridges that you can only get across with your acro bike. Go across them, and there are two suspicious looking patches of grass. "Talk" to them, and it says "If this clump of grass can be moved, it might be possible to go inside."
Well, how do you go inside? And what exactly are you going inside of? I'm completely lost here!
If this has been posted before, I'm sorry.