Uta World

The Uta World is an alternate dimension that is accessed through the Uta Uta no Mi. The user of the Uta Uta no Mi can forcefully transport anybody who hears their singing into the dimension.

The Uta World can only be entered by listening to the user of the Uta Uta no Mi singing. Doing so will cause the receiver to fall asleep, with their consciousness transported to the Uta World. They will be unable to leave, being trapped until one of a few specific scenarios is achieved.
 * 1) The user of the Uta Uta no Mi releases those inside, transporting them back to their bodies
 * 2) The user of the Uta Uta no Mi falls asleep, cancelling the Devil Fruit's abilities
 * 3) Tot Musica is summoned and defeated in both the Uta World and real world at the same time
 * 4) *However, if those inside the Uta World take too long to defeat Tot Musica, they will become trapped permanently, with only the second possibility staying viable.

If the user of the Uta Uta no Mi dies while there are people inside the Uta World, they will become trapped forever, permanently unable to leave.

The Uta World also takes priority over other sub-dimensions, such as the Mirro-World and the Doa Doa no Mi's Air Door. While other sub-dimensions are still accessible from within the Uta World, they cannot be used to access the outside world, instead only connecting back to the Uta World.

Layout and Locations
The Uta World perfectly mimics the world around it, to the point where even those trapped inside don't realize they're been transported. However, the user of the Uta Uta no Mi has complete control over everything within earshot of themselves, allowing them to modify the Uta World as they see fit.

Past
In her childhood, Uta would unknowingly transport the Red Hair Pirates and residents of Foosha Village into the Uta World when she sang to them.

One Piece Film: Red
When Uta held her first concert on Elegia, she took advantage of her popularity to trap as many people as possible in the Uta World. She intended to permanently leave them in the alternate dimension and use her control over it to create a utopia, ending the Golden Age of Pirates and all suffering in the world. However, she was ultimately thwarted by the Straw Hat Pirates, Red Hair Pirates, CP0, SWORD, Big Mom Pirates, and Jellyfish Pirates, forcing her to shut down the Uta World and release those trapped inside.

Trivia

 * While it's stated that Tot Musica is the only connection between the Uta World and the real world, it's shown that Den Den Mushi trapped inside can still broadcast outside; Kenbunshoku Haki is shown being able to penetrate it as well.