In the MCU, the place that exists way down below the size of atoms themselves is known as the Quantum Realm, a dimension that is connected to the core Marvel reality, but where time and space move differently.
The concept is so key to the MCU it's referenced in the title of Ant-Man and the Wasp: Quantumania, which takes Scott Lang, Cassie Lang, and Hope Van Dyne deep into the Quantum Realm.
But to get there, they have to travel through some other parts of the wider Marvel Universe - including the realm known as Sub-Atomica, which Janet Van Dyne namechecks as one of the layers of reality that separate the core MCU from the Quantum Realm.
In Marvel Comics, Sub-Atomica is the name given to a galaxy that exists within the Microverse (the Marvel Comics term for the Quantum Realm), and it's home to a variety of worlds and characters, some of whom - such as Bill Murray's Krylar and Katy M. O'Brien's Jentorra - have cameo roles in Ant-Man and the Wasp: Quantumania.
Interestingly enough, Jentorra has a deeper connection to Sub-Atomica in Marvel Comics, where she's the niece of Jarella, empress of the subatomic world of K'ai. Jarella was the one-time lover of none other than the Hulk, who once had an extended stay in Sub-Atomica himself.
That also makes the Marvel Comics version of Jentorra the cousin of Hulk's son Skaar, who was recently introduced in the MCU at the end of She-Hulk.
Sub-Atomica is also home to Fantastic Four villain the Psycho-Man, a Microversal mad scientist who uses technology that can manipulate the emotions of others to try and conquer the Marvel Universe at large (pun intended).
It's hard to say whether Sub-Atomica or its other denizens will play a larger role in the MCU after Quantumania, but given the fact that Janet Van Dyne indicates in the film that both Sub-Atomica and the Quantum Realm itself seem to be connected to the mysterious realm known as the Void, as seen in Loki, it's possible that Sub-Atomica could become a much bigger deal on the road to Avengers: Kang Dynasty.
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