You just need one of the Pokemon in your party to know the move Strength.
teach it to a pokemon, then walk up to the boulder you want to use and press A
With the HM move strength.
you have to get strength
you need strength (HM4). i cant remmbr wer 2 get it from though. sorry.
Use strength to move all the boulders through the hole then get to the middle
You can use it in most caves to move the big grey boulders out of your way.
A Pokémon needs the HM move of Strength in order to move the big boulders. You can find the Strength HM in the Rusturf Tunnel.
Strength.
You will need HM strength, which you can get in Rusturf Tunnel. To use it though you will need to beat Flannery in Lavaridge Town.
use strength
You teach a Pokemon strength.
Use Strength. It can be obtained at Olivine City in a building next to the Pokemon Center where there is a sailor inside. Teach your Pokemon Strength so that you can push aside big boulders.