From Bulbapedia, the community-driven Pokémon encyclopedia.
Krokka Tunnel (Japanese: クロッカトンネル Krokka Tunnel) is a cave in Fiore that links Ringtown and Fall City. It is the location of the first and fifth numbered missions in Pokémon Ranger. The western exit emerges in Lyra Forest while the eastern exit emerges in East Road. Krokka Tunnel is linked to Panula Cave.
A memorial to Dugtrio, said to be a loyal companion and coworker in the long and difficult tunnel project, can be found near the Lyra Forest entrance. The Hariyama Dojo can be found deep in the cave, containing strong Fighting-type Pokémon.
The Krokka Tunnel is first entered in the first numbered mission of the game, escorting Professor Hastings to Fall City. However, the Go-Rock Squad attack and create rockfalls that prevent residents of Ringtown from visiting Fall City. In the fifth mission, these rockfalls are removed by searching further in the cave for some Hariyama. The Go-Rock Squad are also there, searching for a legendary Pokémon, with the mission ending against Tiffany and her Camerupt.
In the first Ranger Net mission, Lunick and Solana enter the tunnel to capture a Deoxys.
Registeel can be found within the Tunnel's depths, however a bit of work must be done to get to it. Its chamber is guarded by numerous boulders, at least four of which must be crushed for access, along with two places where players must use the Field Move Cross.
This is a listing of the Pokémon found in Krokka Tunnel after the credits: