Question

It seems usefull, but I have no idea where it is or if you can even get it...

Comments

useful it is one of the most awesome moves ever!

Answer 1

You can find TM 26(Earthquake) in Wayward Cave or you can purchase one at the Battle Frontier.

Here's a video where a guy shows you how to obtain it.
http://www.youtube.com/watch?v=Em_3UIpMc04

Answer 2

Deep in the wayward cave i would suggest a pokemon with flash go in to the entrance to the left or else you will be in the wrong one or the battle frontier for 80 bp

Answer 3

Go to Wayward Cave (look here: http://pokemondb.net/pokebase/11418/in-diamond-how-do-i-get-gabite ) and at the very end you will find a small room with a pokeball with TM 26 Earthquake!