To be specific, if you go all the way to the left side under the cycling road, walk 2 steps to the right, then you'll be litterally under the cycling road. Then go up until you reach a cave. You will NOT need Flash, it'll just waste time. You will now be in a small cave with 5 boulders to use strength on. Use Strength on the boulder right in front of you when you enter. Then go to the staircase to the right. Now you should be in a bigger cave that doesn't need Flash. Now you just ride your bike around a few obsticles. You will run into some pretty usefull items along the way. Keep riding until you see a wild gible. There is a secret cavern when you get to the end. TM Earthquake should be in there. So after your tough journey through the hidden cave, you have a gible AND TM earthquake. I'd say that's a pretty good deal! For more detailed walkthroughs, try Youtube.