Instructions
Take Route 44 to Mahogany Town and head east. You will encounter a trainer here, whom you must defeat to continue.
Choose one of the two paths here when the road forks. If you head up you will pass through the mountains. Go down and take a bridge over the water. Either path will require you to defeat a handful of trainers to continue on to Blackthorn City.
Enter the ice cave you encounter after the two previous roads converge. You will need a Pokemon in your party with the Strength ability to make it through this next section.
Pass the puzzle on the ground floor of the ice cave by sliding along the ice to reach the exit. First, go up, then left, up, left, down, left, up and right. Take the ladder down to the next level.
Push the boulders into the holes using the Pokemon with Strength on the next ice cave level to descend one more floor.
Go down the path on the second basement floor and then climb a ladder back up to the first basement level.
Follow the path on this level to reach another ladder that will take you back up to the ground floor.
Take the path out of the ice cave to reach Blackthorn City.