Quantum Realm

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

History
Access to the quantum realm was realized by scientist Henry Pym through use of Pym Particles, particles of 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. Janet van Dyne, Pym's wife and the first person to bear the mantle of the Wasp, disappeared into the quantum realm after sacrificing her life to disable a Russian nuclear missile. It is unknown if she is still alive or not.

During his battle with Darren Cross, Scott Lang was briefly left trapped in the quantum realm, but managed to escape by altering with the Ant-Man suit's mechanisms.

Trivia

 * The Quantum Realm is an adaptation of the Microverse, which, in the comics, was 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.