The Vast: The World Inside the Space Stone
The world inside the Space Stone is simply known as the Vast. It's an abstract realm of endless emptiness, overwhelming to almost anything that enters it. Even the Sleepwalker, a being used to the infinite nature of the Mindscape, is left reeling by it. This dimension is one of the ones the Sleepwalker spends the least time in; there is, after all, nothing at all to interact with in this abstract realm.
The only thing you can do when trapped inside the Space Stone is find your way out; and even that is only something a being who can manipulate space and time, like the Sleepwalker, could accomplish.
The Ellipsis: The World Inside the Time Stone
The world inside the Time Stone is another abstract one, thus difficult to interpret. To be inside the Time Stone is to be subject to the ravages of time in a way the Sleepwalker, a being who's never been constrained by time, could never have imagined. Although he was only inside the Time Stone for minutes, for the Sleepwalker it felt as though centuries had passed. To escape the Time Stone, you have to master its forces sufficiently to create a perfect moment in which it is possible to jump into the next Stone World. Ironically, once you have managed to create such a way out, it has always existed. Such is the nature of time inside the Time Stone.
The World Pool: The World Inside the Reality Stone
The World Pool is perhaps the most fascinating of all the different realms inside the Infinity Stones. Presumably named in honor of Deadpool, it's essentially a vast filing cabinet in which all the adventures of the multiverse are represented as comic books. The World Pool is maintained by a cosmic being known as Archivus, who essentially acts as superintendent to the World Pool. She's less than impressed when Sleepwalker mentions needing to get into the Soul World. "Oh, like from Infinity Wars," she asks, and hands Sleepwalker a copy of the first issue. "Let me know if you need the tie-ins."
Matters then go pretty wild for the Sleepwalker, with the Guardian of the Nexus of All Reality - normally the Man-Thing - dragging the Sleepwalker inside one of the comics. The Sleepwalker experiences a disorienting moment in which he experiences countless different realities - astute readers will recognize several from Marvel's What If? comics, and Killraven's timeline. The Sleepwalker is particularly vulnerable to the World Pool. As the Guardian explains, "You are a creature of imagination, but this is a place of idea given form! All but the strongest are destroyed by the "Reality Force" within the Stone!" It's only with the help of the Guardian that it is possible for the Sleepwalker to survive, let alone escape.
It's a smart move for Marvel to tie the Guardian of the Nexus of All Realities to the world inside the Reality Stone. It makes the point that, in the comics, the Infinity Stones actually embody a universal aspect; thus there is a sense in which every reality actually exists inside the World Pool already. When a being possesses the Reality Stone, they are literally master of all that is, and was, and ever will be.
Infinity Wars: Sleepwalker #3 is on sale now from Marvel Comics.