Check the puddle to the right for a hidden Ultra Ball. Just as you are about to climb the staircase to the mountain's entrance, Cheren shows up again. Since you each have a Jet Badge now, he's here for another battle.
When you defeat Cheren, Alder jumps down from the cliff. He was watching the battle, and tells Cheren that what he does with the power gained by becoming stronger is important. Alder gives you each a copy of HM03 (Surf), then leaves to the south. Cheren climbs the staircase and enters Twist Mountain.
Now that you have the ability to cross bodies of water, you can backtrack to a few locations that were previously unreachable.
The Southeast Ocean
There isn't much of thing to do here (except getting items that previously were blocked by Cut trees if not done already) and that rustling grass appears here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
Route 1 (southwest)
To reach Route 17, you need to get back to Route 1 and surf west. Leave an open slot in your party for the treasure hunter's Egg! Pick up the Max Ether near the western shore, then battle the two Pokémon Rangers and the Fisherman. Grab the Pearl to the southeast, and walk through the gate to reach Route 17.
The swift ocean currents can make travel difficult, so proceed carefully. Surf to the first spot of shallow water, then grab TM06 (Toxic) from the next. Ride the northernmost of the two westbound currents to fight Swimmer Wright and pick up a DeepSeaTooth. Take the southern current west to reach the large island on Route 18.
Climb the staircase to the top of the cliff and battle Hiker Jeremiah. Trek through the tall grass to reach the only building on the island; a girl inside will restore your Pokémon's health. Talk to the Pokémon Ranger here to receive an Egg he found in the Relic Castle. The man in black is looking for a Rare Bone; wild Crustle sometimes hold them, which you can sell to him for 10,000.
If you have a Pokémon that knows Strength, you can cross the bridge to the south and drop the boulder to find a Max Elixir. Go down the staircase and shove the second boulder east to fight Backpacker Sam, and pick up a Dragon Scale. Head to the northwest, then double-back through the tall grass on the island's west side to find HM05 (Waterfall)! Go north and climb another staircase to fight Veteran Ray, then continue east across a second bridge and climb down the last staircase.
Ride the southernmost of the two eastbound currents to wind up in front of a small island. Grab TM24 (Thunderbolt) to the south, then check out the P2 Laboratory. The building is devoid of people, but is cluttered with machines. Surf southwest between the currents and around two Swimmers to reach the large island's interior. A Heart Scale lies on a secluded part of the beach to the south, while Battle Girl Hillary and TM19 (Telekinesis) are farther west.
The next place to visit is Mistralton Cave, situated across the river on Route 6. Somewhere in its depths, the legendary Cobalion lives in hiding. Legends claim that Cobalion saved the Pokémon of Unova from a sea of fire in the distant past. Remember to bring a Pokémon with Strength; Flash is also good to have.
Go south and push the boulder to the hole in the floor. Walk east past Hiker Hugh and move the second boulder to the next hole. Head north along the east wall to fight Hiker Clarke, and pick up the Max Repel. Grab the Hyper Potion to the northwest, and move the third boulder into place to create a shortcut. Climb the stairway to the second floor.
Mistralton Cave 2F
Head south to move the next boulder. Walk across the rock ledge, and double-back along the southern wall to find a Hard Stone. Go north, then west to push two more boulders into place. Head to the northeast, then follow the north wall to the west along the upper level for TM80 (Rock Slide). Go west along the lower level to reach an Iron, then jump south over the ledge, and climb up to the third floor.
Head west from the stairway to find a Revive. Backtrack to the east and follow the wall north to get a Dusk Stone. Talk to the old man to hear a story of the three legendary Pokémon, Cobalion, Terrakion, and Virizion.
The old man's story
In the distant past, when Pokémon and people lived in different worlds, a war between people started an intense fire in a forest where many Pokémon lived. The Pokémon were surrounded by smoke and flames. They were in big trouble. And then, the legends Cobalion, Terrakion, and Virizion appeared! Terrakion used its amazing strength to destroy rocks blocking the escape route. Virizion used its quick movements to protect Pokémon from looming flames. And Cobalion led the frightened Pokémon out of the burning woods. ... The people who started the war were routed by the power of the legends. Because of Cobalion, Terrakion, and Virizion, the war was ended. ... When people fight, there's no peace for Pokémon, either. The three legends learned that the deeds of humans could lead to dire consequences for their world. ... They cut themselves off from people and disappeared. No one saw much of them after that, and the three faded into myth...
Pick up a Rare Candy to the northwest, then go south and drop the final two boulders into position. Head northwest to find Cobalion.
Cobalion's Ability, Justified, gives it an Attack boost whenever it is hit by a Dark-type attack. When this happens, its Sacred Sword attack becomes very powerful, so be careful. Lower its health with Normal-, Grass-, and Ice-type moves before attempting to capture it. If you need to use TM's, it would be recommended to teach a Pokémon Reflect since it can reduce the damage from Cobalion's attacks by half.
The old man approaches you after the battle, and says that because Cobalion is starting to trust you, Terrakion and Virizion will give you a chance to battle them, as well. Terrakion is hidden deep in Victory Road, out of reach for now. Virizion, however, is waiting in Rumination Field, on the eastern side of Pinwheel Forest.
Regardless of the way you entered, look for a hole in the fence northeast of the bridges. Trek east through the dark grass and go to the far southeast edge of the forest to find TM22 (SolarBeam). Go west to reach the small pond, and check for a Rare Candy on the southeast side. Surf west across the water to grab the SilverPowder from the ledge, then sail back across the pond and head northeast. Walk through the fallen log and pick up the Hyper Potion from the tree stump. Rumination Field lies to the north.
Rumination Field is a tiny, isolated part of the forest, with no other point of interest but Virizion itself. Like Cobalion, its JustifiedAbility boosts its Attack when it takes damage from a Dark-type move. Weaken it with super-effective moves (excluding Flying-type moves due to Virizion suffering a 4× weakness due to its type combo), then whittle its health down with Water-, Grass-, or Ground-type attacks.