From Bulbapedia, the community-driven Pokémon encyclopedia.
The Onix Tunnel (Japanese: イワークの洞窟 Iwark's Cave) is an anime-exclusive location in Johto located between Azalea Town and Goldenrod City. It was featured in the episode Tunnel Vision.
The Onix Tunnel is a cave that possesses many stalagmites and stalactites, some of which drip water. Many Onix can be found there, and thus served as obstacles preventing further travel through the cave in much of the episode. However, upon recovering the marker-microphone, Jigglypuff sang the Onix to sleep, Brock later deducing that the echoing properties of the cave probably made the song stronger. The group travels through the cave, Team Rocket attempting to travel through it as well but inadvertently awaking the Onix. Subsequently, Team Rocket finds themselves in some trouble, Jessie even wishing that Team Rocket was blasting off.
Pokémon seen in Onix Tunnel