The Enigmatic Aquarium is a dungeon in the underground of Impact Island which is unreachable until you recruit Blue Pikmin. It appears to be the remains of some sort of marine biology museum, with multiple aquariums hosting a variety of bizarre underwater foes spread throughout the dungeon in large tanks. It is also an extremely creepy and unnerving place. Sometimes, you'll even be forced to enter some of the tanks to progress deeper into the dungeon. You'll be very thankful for the map here.