From Bulbapedia, the community-driven Pokémon encyclopedia.
New Mauville is a power plant located underneath Mauville City in Hoenn. Due to its unique location, the complex is reached only by Surfing from Route 110 east under the Seaside Cycling Road to a small island east of the overhead path. Though the location is not part of the main storyline in any Generation III game, it does play home to a small side quest as well as several items and wild Pokémon, mostly of the Electric-type, making the location similar in style to the Power Plant in Kanto.
After defeating Norman in Petalburg City, Wattson stands at the conceivable intersection of Route 110 and Route 118 in the center of Mauville City. Upon talking to him, Wattson requests that the generator in New Mauville be shut down as a safety precaution. To allow the trainer access to the underground plant, Wattson gives the trainer the Basement Key, which will open the door to New Mauville. After shutting off the generator, Wattson remains in the center of Mauville, ready to hand out TM24 (Thunderbolt) as a reward.
Throughout the plant, there are green and blue doors that only open when the corresponding button color is pressed. Said buttons are found on the ground throughout the facility. The two types of doors cannot be open simultaneously, however. Also, like the Power Plant in Kanto, some "items" on the ground are actually Voltorb and Electrode in disguise.
In the manga
In Pokémon Special, New Mauville was designed by an ancestor of the Trick Master to combat the foreseen overpopulation of children in Mauville City, which was already starting to happen by the Hoenn saga. While its location was not known, Sapphire accidently came across the entrance and entered with Wattson and the Trick Master. After saving Wattson from the power generator, the power was turned on to reveal an enormous playground designed for children. It was quickly put into use.
During these events, Sapphire obtained her Donphan and a Dynamobadge.
It was used for sheltering refugees during the Groudon/Kyogre crisis later in the saga.