If you done know what it is, it's a machine ( a ship to be precise) created by 3 legendary shinigami's.

Former 12th Division Captain, Alfred Alstein
7th Division Shogun, Hajime Yukimura
Former 8th Division Captain, Minamoto Chiruka (deceased)

Hueco Arca was made because of Hueco Mundo falling under the hands of the Anarchist(a short lived clan of rebel shinigami's who invaded hueco mundo and exiled the hollows into the living world.)

With the balance in peril, gotei 13 made a peace treaty with the hollows, Gotei 13 was to help the hollows push the Anarchist out of Hueco Mundo, but with the current force Gotei 1 had at that time, it was imposible, so they build a ship to cater as new home for the hollow race, the Hollows Arc, Hueco Arca.

3 major parts made its completion possible, 1 power, 2 technology and 3rd was structure. But with its power source revealed to Alfred Alstein, the opening took a turn for the worst, Alstein stole the arc.

With the arc gone, Gotei 13 saw it as a form of rebellion, a breech of conditions, so the captain and commander of that time gave the order to eliminate the entire hollow race, sent the best of the best, and lost them both at the same time, only few survived, but from the hollow's side ZERO.

To there own surprise the balance was not affected, the world did not end, so Hanzo and Chiruka investigated further within Hueco Mundo's replicated world, but found nothing.

The extinction of the hollow race was kept from everyone's knowledge, all, but few knew of the real truth.


Centuries had past since the extinction of hollows, to keep with the act, 12th Division created living creatures similar to hollows, one who fed on souls, but it was not later that Arrancars appeared, and they were the real deal and it alarmed those who knew.

At first they acted like they didnt know anything, but a secret kept will soon be revealed, it came to the point where they needed to explain the extinction of hollows, and that the shinigami's existence to date was a joke.

Continuously battling this arrancar like beings, they discovered things, but only really knew what they were after a Magi's explanation.

They were shinigami's inner hollows set free originating from gotei 13's elite captains and vice captains of the said time. Which explained how the Vizards lost there hollow abilities and there masks at the same time.

The hollows within them were stolen, set free, by a demon named Ciel.

Once everything was cleared up, the Vizard Commander knew exactly what to do, and that was to ambush this beings, they were invited to an event no one wants to miss, the day where Hueco Arca was to be re-opened.

And so they battled once more, against the beings who called them selves sins, one event lead into the other, one of which the sudden resurrection of the hollow race. No one knows for sure how they came to live once more, but they were more civilized compared to before, but fewer.

The rips during the gate's opening were so strong that it changed the rules of hollowfication, the Vizards couldn't regain there mask, and the answer lies within Hueco Arca which fell and vanished.