In Omega Ruby & Alpha Sapphire, Ampharos’ Mega Stone Ampharosite can be found on the ground in the northeastern back section of New Mauville. This is an underground power plant located near Mauville City.
However, you aren’t allowed in without Wattson’s permission.
So if you want access to New Mauville, you’ll have to complete Wattson’s sidequest that’s only available after you’ve defeated/captured Kyogre or Groudon.
Most Mega Stones, including Ampharos’ Mega Stone, are only obtainable after you defeat/catch Kyogre or Groudon in Sootopolis City; more specifically, in the Cave of Origin.
This major event happens right before you’re able to challenge the eighth and final Gym.
So after you defeat/catch Kyogre or Groudon, head to Mauville City’s Pokémon Center. Wattson will wait for you in front of Square Tower if you want to begin the sidequest that unlocks New Mauville.
Additionally, you will need a Pokémon with Surf to access New Mauville.
Note: If you already have access to New Mauville, you can skip to Step 13 of this guide to see the Ampharosite’s exact location.
Ampharosite Location (Step-by-Step)
Step 1: From the Mauville City Pokécenter, head south and talk to Wattson.
Step 2: Then go back inside the city’s interior using the north entrance.
Step 3: Walk west until you see an elevator on the north wall.
Step 4: Use the elevator to get to the second floor.
Step 5: Make your way toward Wattson’s apartment, located on the city’s south side.
Step 6: Follow the hallway heading south.
Step 7: Then you’ll want to right and continue moving east down the hallway.
Step 8: Enter the second apartment from the left (it’ll be on the northern wall, pictured below).
Step 9: Talk to Wattson here, and he’ll grant you access to New Mauville.
Step 10: Then go to Route 110 using Mauville City’s south exit.
Step 11: Use Surf on the body of water near Cycling Road’s entrance. Move east to reach New Mauville.
Step 12: If you keep Surfing east then you should land outside of New Mauville’s entrance.
Step 13: Interact with the gate to enter the power plant.
Step 14: Once inside, go through the door in the northwest corner from your current location.
Step 15: In the next room, go up and then turn right. You’ll want to go towards the rightmost part of the room.
Step 16: At the end of the hallway, interact with the sparkling item on the ground in the small nook below you. This is the Ampharosite.
Is Mega Ampharos Good?
Mega Stones like the Ampharosite are held items that allow certain Pokémon to Mega Evolve, granting them a massive boost to their base stats.
Some Pokémon also change types upon Mega Evolving. Ampharos, after Mega Evolution, becomes an Electric/Dragon-type Pokémon.
Most Mega Evolutions also change the Pokémon’s ability, which lasts until the battle concludes.
Mega Ampharos gains the ability Mold Breaker, which makes the user’s moves and their effects ignore other Pokémon’s abilities. Though this ability isn’t particularly useful on Mega Ampharos.
But as a whole?
Mega Ampharos has an excellent Special Attack stat, and great bulk for an attacker. However, its low base Speed and limited movepool prevent it from being more commonly used in competitive battles.