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.
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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.
I don't know if it's the same as in Sapphire but I think you have to go to Mauville City and get the Basement Key off the gym leader there, Wattson, then go down to wear the man is fishing. Use surf there and go right. You will come across 2 islands one that has a tree and a sign saying something like 'Team Aqua was here, but scribbled oover was, Team Magma Rules!' Surf some more and on the next island there is a metal door, and that's where you use the basement key.
After a certain point in the game, you will see Watson standing in the middle of Mauville outside of his gym. If you talk to him and agree to help him with his problem, he will give you the key to New Mauville.
The cave Wattson, the Mauville City Gym Leader, tells you to venture inside is New Mauville. He wants you to turn off the generator inside New Mauville. It appears to be acting up and causing problems for people in Mauville City. Inside New Mauville are various items and switches that allow you to navigate through New Mauville and reach the generator. There is also wild Pokemon in the power plant like Magnemite, Voltorb (Three fake items are Voltorb), Electrode, and Magneton. To reach New Mauville you must head south of Mauville City and use Surf in the lake located in Route 110. Surf below Cycling Road and you will see a cave entrance. That is New Mauville. To enter the power plant, it requires the Basement Key (Which Wattson gives to you after taking the request). Hope this helps.
1st of all it's just like emerald you get 5 badges and go to mauvill and outside will be wattson and he will give you the key and WALA go get to go on a vacation to N_E_W M_A_U_V_I_L_L!!!!!!!!! =D