From Bulbapedia, the community-driven Pokémon encyclopedia.
The Trapinch underground labyrinth is an anime-exclusive location underneath a grassland reserve on Izabe Island in the Hoenn region. It was featured in Beg, Burrow and Steal.
The area is largely unknown as it hidden deep underground. The confusing network of tunnels and its tendency to experience underground flooding make it particularly dangerous. Because of its shade and water supply, Trapinch like to nest in the labyrinth while they prepare to evolve into Vibrava during a seasonal mass evolution event.
Hal discovered the secret oasis after falling down a Trapinch-dug hole as a child. He went on to dedicate his science career to studying Trapinch and locating the hidden labyrinth. In Beg, Burrow and Steal, Hal found the labyrinth once again, this time accompanied by local Pokémon Ranger Elisa, Ash, and his friends.
Pokémon seen around the Trapinch underground labyrinth