From Mauville City, go south, you'll meet the Fisherman. Surf and keep to east. Then, you reach the New Mauville.
Go south until you are below and out of Mauville City, and Surf in the water until you find a cave: That;s New Mauville.
In Pokemon Sapphire when the player defeats Gym Leader Norman in Petalburg City, Wattson, the Gym Leader of Mauville City can be found at the intersection of Route 110 and Route 118. Upon talking to him, he will give the player the Basement Key to access New Mauville City.
I would say the one on the way to de-activate the generator at new mauville near the cycling rode in Ruby, Sapphire, and Emerald.
First beat the Mauville Gym leader, Wattson.
From Mauville City, go south, you'll meet the Fisherman. Surf and keep to east. Then, you reach the New Mauville.
magnetons cannot be found in the wild, however you can find a magnemite in new mauville just outside of mauville
Go south until you are below and out of Mauville City, and Surf in the water until you find a cave: That;s New Mauville.
At new mauville after you beat your dad go to wattson and he'll give you directions
In Pokemon Sapphire when the player defeats Gym Leader Norman in Petalburg City, Wattson, the Gym Leader of Mauville City can be found at the intersection of Route 110 and Route 118. Upon talking to him, he will give the player the Basement Key to access New Mauville City.
Head south of Mauville, towards the entrance to cycling road. Surf across, and head west. You'll eventually find a cave. That's New Mauville.
you get it in the NEW MAUVILLE basement
I would say the one on the way to de-activate the generator at new mauville near the cycling rode in Ruby, Sapphire, and Emerald.
you can get it after you beat the 5th gym. you go to mauville city and Watson is going to be there. talk to him and he will tell you to shut off t he generator in new mauville. you surf there ans you g=find a thunderstone at the end.
When you can surf, go to mauville and make sure you have a basement key, surf to the cave, go in and unlock the door... You go in the water on route 110 right under the sign for the bike road building.
outside of Mauville. you go south to the the water and use surf if you have it and west. there you will find land that leads to a cave, in that cave is New Mauville use the Basement key given to you by the Gym Leader of Mauville and you are there.( there are some Pokemon that can only found there polemon like Voltorb, it's eveoled form Electrode, magentmite and it's evoled form magenton.
After you beat your dad, go back to Mauville. You will see Wattson standing outside. He will ask you to shut off the generator in New Mauville. After you do that, return back to him. He will thank you and give you the TM thunderbolt.