You have to have the HM Strength which you can find on Iron Island with Riley.
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in Pokemon platinum you need to have (strength) you can obtain it at iron island but you can only get it when you have 6 badges.' check out my new website: http://pokemonds.moonfruit.com
First, you have to get the HM strength, then teach it to and a Pokemon and you can move boulders.
the three lake Pokemon will tell you where to move them by which way they float
Strength
use strength