Quantum Realm

The Quantum Realm is an alternate dimension only accessible through tremendous shrinking.

History
Access to the quantum realm was finally realized by scientist Doctor Hank Pym, through the use of Pym Particles, particles of an extradimensional nature, that are capable of altering the size of beings or objects; if shrunk to a small enough level, objects whose growth is manipulated by the particles may gain access to the quantum realm, where space and time are believed to be irrelevant. Janet van Dyne, Pym's wife and the first person to bear the mantle of Wasp, disappeared into the quantum realm after sacrificing her life to disable a Russian nuclear missile. Following this event, Pym dedicated himself into researching the quantum realm though he was unable to generate concrete results or information.

During his battle with Darren Cross, Scott Lang was briefly left trapped in the Quantum Realm, but managed to escape by altering the Ant-Man Suit's mechanisms, replacing the shrinking regulator with an enlarging one from a weaponized Pym Particle. Lang claimed to have no recollection of what he experienced there, however, much to Pym's dismay.

Nature and Structure
The Quantum Realm is said by Pym to be a largely unexplored dimension, of which scientists currently know practically nothing about. He also says the concepts of time and space become irrelevant when entering the realm, and whoever tries to explore it may become trapped "for eternity."

During Scott Lang's incursion, we see it begins on a microscopic level, then entering atomic size, and latter dissolving into pure energy and waves, and ending in a fractal-like reality where the shrinking person meets an infinite number of mirror images of themselves (a possible indication of the "soul" concept).

Trivia

 * In the comics, the Quantum Realm is known as the Microverse, which consists of many different dimensions all accessible through the same means as they are in the Marvel Cinematic Universe. Notably, the Microverse was also home to the Micronauts, a superhero team.
 * Wasp also was here during the time of her supposed death during the Secret Invasion, before being found here by Hank Pym.
 * Marvel Studios cannot use the name "Microverse" as it is packaged with the Micronauts film rights that are with Paramount.
 * The depiction of the Quantum Realm in Ant-Man is based on the former Disney attraction.