From Bulbapedia, the community-driven Pokémon encyclopedia.
Marine Tube (Japanese: マリンチューブ Marine Tube) is an underwater location in Unova introduced in Pokémon Black and White 2. It is a tube that connects Undella Town with Humilau City under Undella Bay.
Known as "the walk-through aquarium", the Marine Tube was built during the two years between Pokémon Black and White and their sequels. It was constructed to commemorate the friendship between Undella Town and Humilau City and was designed, as stated by a NPC at the Humilau entrance, "to see, not to move." When the player first arrives in Undella Town, the Marine Tube is closed for cleaning and yet to open. The cleaning is completed at the same time the player earns his or her seventh badge.
The Marine Tube is a transparent tube made of 9.8 inch glass built to withstand high water pressures and contact with huge Pokémon. The tunnel was made above ground and lowered into the water. Crossing a six-foot-deep ocean trench, the Marine Tube is one of the many pinnacles of technology in the Unova region.
Rocks covered with moss and pyramid-like rocks can be seen in the water surrounding the tube. Lights have been placed outside the tube to provide visibility. There is a canyon halfway along the tube's route, the bottom of which cannot be seen. The Water-type Pokémon Alomomola, Basculin, Frillish, Mantine, and Wailord will frequently swim past. Some Pokémon can only be observed at certain times of the day; the player can get the Wailord Watcher medal by spotting an elusive Wailord between 3-4 p.m.
There are no wild Pokémon or Trainers to battle in the Marine Tube. There are no items lying around, although the self-proclaimed first fan of Roxie will give the player a Black Sludge if they show her a Pokémon holding Poison Barb.