It is sort of hidden. Starting from the north end of Route13, make your way south. Once you reach the part where you go down the steps into the really tall grass (the one where you can have double battles). Make your way right until you reach water. There should be a man standing there. Then you need to surf out around the rocks there and travel north along the coast.. Then you are there. This is where you can catch the legendary Pokemon, which is in the inner cave. I do not know haw to get to the inner cave though.
****You have to have the National pokedex to have access to the giant chasm.
giant chasm
it is at the end of giant chasm
At the giant chasm
from undella town you follow the path upwards and move the strength boulder you can get to lacunosa town from here as well as giant chasm
he is in the giant chasm
Kyrum
In the giant chasm
You go to giant chasm
in giant chasm
Giant chasm
Giant Chasm
giant chasm
it is at the end of giant chasm
After you beat the elite four you can catch it in the giant chasm there is also a legendary Pokemon in the giant chasm called kyurem.
in the giant chasm
In the Giant Chasm when you get up to that part
At the giant chasm