After Kay developed multiple personalities to deal with her childhood trauma, the personalities were represented as inhabiting stops along a subway line, with one personality named Driver 8 acting as the conductor. "The Underground" as they called it determined which personality was in control of Ka
The Underground is the metaphysical representation of Kay Challis' fractured mind.
After Kay developed multiple personalities to deal with her childhood trauma, the personalities were represented as inhabiting stops along a subway line, with one personality named Driver 8 acting as the conductor. "The Underground" as they called it determined which personality was in control of Kay's body at any given time, with the primary controller being Crazy Jane.
When Jane was sent into a coma, fellow Doom Patrol member Robotman volunteered to find her within her mind. Rebis used their power to send Cliff into Jane's head and explored the Underground, meeting all of Kay's other personalities. Cliff was forced to fight a representation of Kay's hatred of her father before saving Jane from her coma. 
When Jane was suppressed within the Underground by an evil personality named Dr. Harrison that had created a cult and a gene bomb, the Doom Patrol discovered her in a location called Janestown and tried to free her. Once again, Negative Man used his power to send Cliff to the Underground and talked to Jane. There, she explained her plan to destroy Dr. Harrison, which involved letting the gene bomb explode to eradicator her. When the plan was a success, Cliff returned to the real world. 
In an alternate future, Jane was trapped inside the Underground when Casey Brinke - an imaginary person - was sucked inside Kay's body. Casey, now going by Jasey, couldn't find Jane, believing she was gone forever.
However, when Jasey discovered Mento's plan to go back in time to ensure that he could kill Robotman to save the world, Casey retreated into the Underground, hoping to find Jane's memory of the past and warn her of the impending incident. 
In the present, Casey reached Jane's stop and successfully warned her about Mento, avoiding the possible future. 
- Crazy Jane
- K-5: The original Kay Challis, who vanished at age 5. She is "sleeping" in one of the lower stations of the Underground.
- Miranda: The former dominant personality. She destroyed herself after the church incident.
- Liza Radley: A normal personality awakened as a result of a loving environment, who pushes Jane to recovery. The other personalities are unsure of how to react to Liza and feel threatened by her. She is named after the B-side of the Jam's single "Start!"
- Daddy: An impression of Jane's father, Arthur Challis, Daddy appears as a giant monster made of insects, excrement and puzzle pieces. Daddy talks with Jane's voice. It was destroyed.
- Driver 8: Conductor of the Underground subway, named after the R.E.M. song. The Driver's hat has an infinity symbol (∞) on it.
- Black Annis: An aggressive misandrist, equipped with sharp claws, red eyes and blue skin.
- Baby Doll: A childlike personality that believes everything is lovely.
- Scarlet Harlot: A nymphomaniac with the power to create ectoplasmic projections and absorb stray psycho-sexual energy.
- Baby Harlot: An integration of Baby Doll and Scarlet Harlot.
- Penny Farthing: A stuttering, nervous personality.
- The Hangman's Beautiful Daughter: An artist with the power to psychically activate her own paintings; whose name is taken from the title of an album by the Incredible String Band. After Jane and her alters found harmony with each other, she was able to animate art that she had not created herself.
- Rain Brain: She speaks in stream of consciousness and can take on an abstract immaterial form.
- Flit: She can teleport anywhere. Dresses in late 80s fashion.
- Mama Pentecost: An expert enigma and cryptogram solver.
- Sun Daddy: A gigantic figure with a sun for a head with the power to throw fireballs.
- Sex Bomb: A female personality that explodes when sexually aroused.
- Stigmata: She bleeds from her hands and feet and relives the church incident endlessly.
- No One: A personality that is very aggressive. It was able to sense the Fifth Horseman and the Painting before it was activated.
- Lucy Fugue: She has radioactive bones and see-through skin. She can also generate harmonic vibrations, a power she used to defeat the Antigod.
- Hammerhead: She is very aggressive towards everyone.
- Spinning Jenny: A personality that is prone to panic attacks.
- Flaming Katy
- Lady Purple: She can see the future but rarely speaks; her name possibly comes from Siouxsie and the Banshees' song "Christine."
- Merry Andrew: Dresses as a Harlequin and carries toys.
- The Sin-Eater: She believes she must suffer for her sins. Jane brings her out as a defense when being tortured.
- Jill-in-Irons: She is wrapped in large chains.
- The Secretary: A neat and orderly pessimist who rarely shows emotion.
- The Weird Sisters: A three-in-one personality. Depicted as a decrepit old woman with three heads. Their eyes are milky white. Her station is that of a carnival fortune teller.
- The Engineer: He assists Driver 8 in maintaining The Underground.
- Kit W'the Canstick: An old woman who carries a burnt-out candle.
- Jack Straw: A living scarecrow.
- The Pointman: He assists Driver 8 in maintaining The Underground.
- Sylvia: She bears Jane's feelings of claustrophobia. She is locked inside of a small room, reciting poem fragments. She believes if she can put the fragments together she can use them as a key out of the room.
- Butterfly Baby: Constantly suffers pain on a Hellraiser-like level in the deepest part of Jane's mind.
- The Shapeless Children: Constantly repeat "Daddy don' do it."
- Bizzie Lizzie Borden: Jane's ninth alter ego, who may not be real.
- Pepper's Ghost
- Driller Bill
- Pretty Polly: Wears a black Victorian dress, X's carved into her eyes
- The Snow Queen
- The Signal-Man
- Though the concept of the Underground would be first explained by Driver 8 in Doom Patrol (Volume 2) #20, it wouldn't make its first appearance as a location until Doom Patrol (Volume 2) #30.