Ajahn Thanissaro wrote:
This dimension is characterized by a type of consciousness that lies outside of the range of the sensory consciousness involved in dependent co-arising and the realm of the six senses — a realm the Buddha calls the "all." Thus it's totally free from suffering.
MN-49 wrote:"'Consciousness without surface, endless, radiant all around, has not been experienced through the earthness of earth... the liquidity of liquid... the fieriness of fire... the windiness of wind... the allness of the all.'"
The canonical image for this sort of consciousness, totally independent of nutriment, is of a ray of light that doesn't land anywhere.
SN 12.64 wrote:"Just as if there were a roofed house or a roofed hall having windows on the north, the south, or the east. When the sun rises, and a ray has entered by way of the window, where does it land?"
"On the western wall, lord."
"And if there is no western wall, where does it land?"
"On the ground, lord."
"And if there is no ground, where does it land?"
"On the water, lord."
"And if there is no water, where does it land?"
"It does not land, lord."
"In the same way, where there is no passion for the nutriment of physical food... contact... intellectual intention... consciousness, where there is no delight, no craving, then consciousness does not land there or grow. Where consciousness does not land or grow, name-&-form does not alight. Where name-&-form does not alight, there is no growth of fabrications. Where there is no growth of fabrications, there is no production of renewed becoming in the future. Where there is no production of renewed becoming in the future, there is no future birth, aging, & death. That, I tell you, has no sorrow, affliction, or despair."
In line with his discussion of rebirth, the Buddha never offered a metaphysical explanation of what this consciousness is or how it might be. After all, it would be a mistake to justify the reality of the unconditioned with reference to the conditioned, as it's not dependent on any thing or any "how" in any way.
However, the Buddha did show how to get there: That's why his image for the practice is a path. A path to a mountain doesn't cause the mountain, but it does provide the opportunity for walking there. The path of practice doesn't cause the unconditioned, but it does provide the opening for attaining it.
The Canon, when describing a person's full awakening, never depicts the accompanying knowledge as touching on "what" or "how" this unconditioned consciousness is. Instead, the knowledge is said to begin with a realization of release from the asavas (fermentations, effluents) of sensuality, becoming, and ignorance (MN 19), along with the realization that that release is once and for all (MN 146). Then it proceeds to a realization of the future implications of that release (DN 29), starting with the fact that it has put an end to any future rebirth.
In the Buddha's own case, he expressed the knowledge like this:
SN 56.11 wrote:"Knowledge & vision arose in me: 'Unprovoked is my release. This is the last birth. There is now no further becoming.'"
The two most frequently used descriptions of the knowledge accompanying the attainment of arahantship make the same point like this:
SN 35.28 wrote:"With release, there is the knowledge, 'Released.' One discerns that 'Birth is ended, the holy life fulfilled, the task done. There is nothing further for the sake of this world.'"
AN 8.30 wrote:Dwelling alone — secluded, heedful, ardent, & resolute — Ven. Anuruddha in no long time reached & remained in the supreme goal of the holy life for which clansmen rightly go forth from home into homelessness, knowing & realizing it for himself in the here-&-now. He knew: "Birth is ended, the holy life fulfilled, the task done. There is nothing further for the sake of this world." And thus Ven. Anuruddha became another one of the arahants.
In other words, when the mind returns to the fabricated dimension after its total encounter with the unfabricated dimension and has realized its release, the realization that it's through with birth/rebirth — on both the macro and the micro levels — is the first thing that spontaneously occurs to it. This realization of the ending of birth leads to the further realization that all suffering has been ended as well.