Maniac Tunnel (Japanese: マニアトンネルManiac Tunnel) is a tunnel that leads between Route 214 and the Solaceon Ruins. Before the digging is complete, it is known as Ruin Maniac Cave (Japanese: いせきマニアのあなRuin Maniac Hole).
The Ruin Maniac (who uses the sprite of a Collector, in spite of his name), when the player first encounters him, will propose a race: he will dig straight through the mountain as the player captures the different Unown. This race, of course, ends in a tie; after the player has captured the 26 Unown that are in the form of letters, the tunnel itself will extend straight through to the previously-inaccessible upper chamber of the ruins, where only the ? and ! forms of Unown are available.
As the player progressively catches the various forms of Unown, the cave will lengthen, and the encounter rates of the Pokémon in the cave will change (specifically, Hippopotas will be more common). Also, when player has 10-25 Unown caught, Ruin Maniac claims he is now known as Digging Maniac (Japanese: あなほりマニアHole Digging Maniac).
The Maniac Tunnel appeared in Halting Honchkrow, in which Pearl hid in the cave while thinking of a way to get past some Galactic Grunts blocking the gate into Veilstone City. To his surprise, he then encountered two Unown, Grumpy and Flaky, who he and hisfriends had met earlier at the Solaceon Ruins. Upon inspecting the cave more closely, he discovered that it and the ruins were connected. He also came to realize that the cave had been formed when Team Galactic used the Galactic Bomb at Lake Valor a couple days earlier. With the help of other Unown from the ruins, Grumpy and Flaky defeated the Grunts blocking the gate, allowing Pearl to enter Veilstone City and the Team Galactic HQ.
The concept of Ruin Maniac Cave/Maniac Tunnel is similar to that of Three Isle Path's concept: a person is trying to dig through a cave, and the player must finish certain conditions for the person to dig further.
This is the only location in the Sinnoh region that wild Hippopotas can be found.