After you go through Route 8, you reach the site of Captain Mallow's trial: Lush Jungle.
Captain Mallow's Trial
Go upward in the jungle to talk to Mallow. She wants you to gather four ingredients for her: Mago Berry, Tiny Mushroom, Revival Herb, and Miracle Seed. She gives you a Forage Bag to put the collected items into. She says to use Stoutland Search to find the hidden ingredients in the jungle.
Go to the left from Mallow and Stoutland can help you find mushrooms where the sun is shining on the ground. Be sure to take the Tiny Mushrooms. After you take them, a level 22 Parasect attacks. You can't catch it during the trial, so just defeat it.
Go north from here, and you reach a crossroads. You can either go left to a new area, or go right through the grass. First, go to the new area to the left.
In this area, go to the left and have Stoutland check on the small tree below the grass to get an X Sp. Atk.
Go farther left and you find a shaking tree. Approach it, and a level 23 Fomantis attacks. After you defeat it, it might drop an item.
Use Stoutland to check the area where you fought Fomantis, and you will find a Mago Berry.
Before you leave this area, you can check the area above the grass to find a Poké Finder spot.
Go back east to reach the previous area. Go through the grass to the right, then have Stoutland help find a hidden X Defense in the grass to the right.
Go farther up in the grass and go to the right to find a Max Revive.
Go slightly northwest from this grass to find a Super Potion.
Now go up to a new section of the jungle.
Go upward, and Stoutland can help you find a hidden Hyper Potion to the right of the sign.
The sign says that Revival Herbs are shorter than the surrounding grass. Go to the northwest of the area and have Stoutland help you find the suspicious grass to the left. Say Yes when asked to pull it up. You will find a Revival Herb, but a level 23 Fomantis will attack. After you defeat it, Mallow will speak to you again.
There is a hidden Heal Powder on the rock southeast of where you found the Revival Herb.
You can crawl through the tree to the left to find TM86 Grass Knot. Then crawl back through the tree to return to the previous area.
Go south, then southeast to a new area. Go to the right and have Stoutland check behind the brown rock to find a hidden X Attack.
Go downward and have Tauros break through the rocks. One of the rocks has a dark spot under it. Have Stoutland check this spot to find the Miracle Seed.
Go left from here and go down the ledge to find a Big Root.
Go down the next ledge and approach Mallow. If you are ready to battle the Totem Pokémon, say Yes. Press the A button repeatedly until the Totem Pokémon appears. It is a level 24 Lurantis, with boosted Speed.
After you defeat the Totem Lurantis, you get the Grassium Z-Crystal, and Mallow gives you 10 Nest Balls.
After that, Professor Kukui gives you TM67 Smart Strike.
The Professor tells you to visit the Dimensional Research Lab when you are ready for your Grand Trial. Before that, he recommends catching Grass-type pokémon here in Lush Jungle, because they will be strong against Olivia's Rock-type pokémon.
Other types that work well against Rock-type pokémon are Water, Fighting, Ground, and Steel. The types that are weak to Rock are Fire, Ice, Flying, and Bug.
When you go to the Dimensional Research Lab, you find Lillie there. After a cut scene, go into the lab, then check on the elevator button and go to 3F.
You meet Professor Burnet. She tells you about the Ultra Wormhole that sometimes opens over Alola. There are legends that the guardian deities of the islands fought against fearsome pokémon known as Ultra Beasts that came from the Ultra Wormhole.
The bookshelves in the lab mention Bronzong, Giratina, and Palkia. It also mentions the Interdream Zone that Burnet has researched.
When you're done investigating, go to Professor Kukui at the elevator. You should go through Diglett's Tunnel next, which goes to Konikoni City.
Press the elevator button and go to 1F, then leave the building. A brief cut scene will occur.
Diglett's tunnel is on the south side of the road, between the Dimensional Research Lab and the Pokémon Center.
Beyond the Rock
In the eastern part of Lush Jungle, there is a big rock. Read this section after you are able to push aside the big rock.
After pushing the rock, you can reach a cave. Inside, you can get a green sparkle for the Zygarde Cube. There is another rock, so push it to the right twice, then up once, then go south to get an Awakening. To the right of that, there is a hidden Rare Candy. Then go north to get TM53 Energy Ball.