After completing the three trials on Akala Island, you go through Digglet's Tunnel on your way to Olivia's Grand Trial.
Go Through Diglett's Tunnel
In the tunnel, go downward and Olivia will check to see which Z-Crystals you have. Then she will let you pass.
Go downward and the path will split.
To the right, you can battle Worker Frank. He has a level 22 Diglett. You get 880 Pokédollars for winning.
To the right of Frank, there is a Dusk Ball.
Go back to the crossroads. To the left, there is a Black Belt who will only battle you after you have defeated all the other trainers in the tunnel.
There is a hidden Escape Rope on the two rocks to the left of the Black Belt.
To the left, there are two rocks around the corner that you can check to get a hidden X Attack.
Go north past the stairs and use Tauros to break the rocks, then go to the left to get a Fire Stone.
Then go back to the stairs and go up. You will encounter two people from the Aether Foundation.
Go farther along the path and you find a Hyper Potion next to a wheelbarrow.
You can go south from the wheelbarrow to find a narrow passageway. Go through the passageway and you find some rustling sand around a rock. You can touch the rustling sand to battle a pokémon.
Continue along this path and you can battle Worker Jeff. He has a level 21 Roggenrola and a level 22 Mudbray. You get 880 Pokédollars for winning.
From there, you can go to the right and jump down the one-way ledge.
Go to the right, past the wheelbarrow, and you can battle Worker Vaclav. He has a level 22 Machop. You get 880 Pokédollars for winning.
In the northeast corner of the area with Worker Vaclav, there is a sparkle that you can collect with the Zygarde Cube.
To the left of the sparkle, there is a hidden Nugget in the wooden crate.
Go south from there to find a couple of Team Skull Grunts. You will battle them both, with Hau helping you. The Grunts have a level 22 Fomantis and a level 22 Salandit. You get 1408 Pokédollars for winning.
After the battle, Hau gives you a Max Ether.
To the right, there is a shortcut that puts you close to the northern exit of the cave. You can use this if you want to go back and battle Black Belt Greg near the entrance of the tunnel. He has a level 23 Hariyama. You get 736 Pokédollars for winning, and Greg gives you a Max Revive after the battle.
Otherwise, you should go south past where the Team Skull Grunts were. The exit goes to Route 9.
There is a police station at the start of Route 9. Go southwest from there and you can battle Police Officer Haruki. He has a level 24 Growlithe. You get 960 Pokédollars for winning.
Go southwest from there and you will see a gate, but don't go through yet. Keep going southwest to a small beach. There is a hidden X Accuracy on the rock on the left.
You can battle Fisherman Mike. He has a level 23 Chinchou. You get 736 Pokédollars for winning.
To the right of Fisherman Mike, there is a Net Ball.
Now go through the gate northwest of here to enter Konikoni City.