Undertown is an underground alien settlement where many kinds of alien species live and thrive below Bellwood.
In The More Things Change: Part 2, Ben first discovered Undertown when he and Rook pursued Bubble Helmet, Fistina, and Liam underground. Ben discovered that Psyphon had been using Undertown as a base of his racketeering operations. Ben ended up fighting Psyphon, Bubble Helmet, Fistina, Liam, Thunderpig, and other unnamed henchmen of Psyphon while fighting Buglizard. After Ben fought off Buglizard, Psyphon and his followers were arrested by the Plumbers.
In Special Delivery, Ben made deliveries for Mr. Baumann to multiple citizens and businesses of Undertown in compensation for unintentionally destroying his property. During which, he ended up unraveling a secret auction held by Psyphon, and foiled his plans.
In The Frogs Of War: Part 1, the town was revealed to humans for the first time, as frightened Undertown citizens launched through the asphalt, and left Earth in search of another home.
According to Derrick J. Wyatt, after the Way Big fight in Destroy All Aliens, a large section of Bellwood was destroyed, an alien labor was secretly brought in to rebuild the city faster than humans could do alone. During this construction, the alien workers built what started out as a very small camp underneath Bellwood. This camp grew over the years until it was the size of a city itself.
- Blarney T. Hokestar's Wagon
- Lair of the Kraaho
- Pakmar's Pet Store
- Blarney T. Hokestar's store
- Alien Salon
- Gas House
- Y-it's Scooter Rentals
- Pakmar's Fine Teas
- The Black Hole
- Rabbit-like Alien
- Members of The Worst's species
- Members of Pesky Dust's species
- Human/Alien Hybrids
- Unknown species
Ben 10: Omniverse
- The More Things Change: Part 1 (first appearance)
- The More Things Change: Part 2
- Have I Got A Deal For You
- Hot Stretch
- Of Predators And Prey: Part 1
- Of Predators And Prey: Part 2
- Many Happy Returns
- Gone Fishin
- Ben Again
- Store 23
- Special Delivery
- Showdown: Part 1
- The Frogs Of War: Part 1
- Rules Of Engagement
- Return to Forever
- According to Derrick J. Wyatt, Undertown has been underneath Bellwood since sometime fairly after Ben 10: Destroy All Aliens.
- As of A Jolt From The Past, Undertown is, at least partly, powered by a volunteer group of Nosedeenians who were freed from imprisonment by Ben and Rook.