The Cluster

"Want to stay... But... Can't stop! Going to form! Can't stop! Going to form! Help! Help! Help! Help..."

The Cluster is a massive Cluster Gem that was first mentioned in "Jail Break" and first seen in "Gem Drill". It is composed of millions of gem shards and, in its emerged form, would be larger than the Earth. It assisted Steven in bubbling itself in "Gem Drill" and remains dormant in the Earth's mantle.

"Jail Break"
"The whole point of coming here was to check on the Cluster!"

The Cluster was first mentioned by Peridot in "Jail Break". Arriving on Earth and checking on it was the primary objective of her mission, with Jasper as her escort and Lapis Lazuli as her informant.

"Catch and Release"
"Look, I know you're scared, but I'm not going to hurt you. And whatever's going on, whatever the Cluster is, I wanna help."

Details were later mentioned in "Catch and Release", when Peridot was cornered and finally captured by the Crystal Gems. When Steven released her, she reveals that the "Emergence" of the Cluster would be a catastrophic event that would destroy everything on Earth.

"When It Rains"
"Open the door, Peridot! If this Cluster is putting us in danger, you need to tell us what it is so we can stop it."

Peridot explains that after Homeworld no longer saw Earth as a viable colony to mass-incubate Gem Warriors, they started doing experiments with the Gem shards, fusing them together to create an artificial fusion as a type of "Gem geo-weapon". She reveals in the Prime Kindergarten Control Room that the forced Gem fusions they were fighting were prototypes for the Cluster, which was revealed to be a giant forced Gem fusion at the core of Earth composed of millions of gem shards. She mentioned that it would be much bigger than Earth if it emerges and takes its physical form, and will destroy the planet once it does so.

"Back to the Barn"
"This is the Cluster. A massive, artificial fusion composed of millions of Gem shards. It has laid dormant for thousands of years within this planet's crust. When this Gem activates and takes its form, the result will be catastrophic."

Peridot mentions the Cluster to the Crystal Gems, revealing the details of what the Cluster actually is. The Crystal Gems then decided to build a drill and try to reach the Cluster before it gets to the end of its incubation period.

"It Could've Been Great"
"Gotta find the right file... aha! Cluster, Cluster where are you, Cluster? Aha! There's the insertion point. Looks like the Beta Kindergarten in Facet Nine. It's the smaller of the two, not nearly as impressive as yours, Amethyst."

Peridot revealed the Cluster's location through the Diamonds' computer in the Moon Base. As shown, it was located beneath the Beta Kindergarten, relative to the Barn and was embedded deep in the mantle approximately 2,500 units down.

"Message Received"
"But the Diamonds are bad! They don't care about the Earth! They wanted to hollow it out, and now they wanna blow it up with the Cluster!"

After embezzling the Diamond Communicator, Peridot argues with Yellow Diamond to terminate the Cluster so that they would not sacrifice the organic life for such a Gem geo-weapon. Yellow Diamond replies, "I want my Cluster, and I want that planet to die. Just make that happen". She also orders Peridot to let the Cluster grow undisturbed.

"Super Watermelon Island"
"The ground shook. This could be the start of the emergence of the Cluster. Stage 1: Slight tremors every quarter hour. Stage 2: Full scale earthquakes. Stage 3: the Earth is destroyed! We're running out of time. We need to drill right now!"

The Cluster tries to emerge in this episode and causes some tremors. Peridot recognizes these tremors and says that the Cluster is causing them, but the other Gems believe that Malachite is causing them. After they defeat Malachite and force her to unfuse, the Cluster causes an earthquake on Mask Island, which results in Jasper falling into a fissure.

"Gem Drill"
"Ugh! Am I the only one who understands the meaning of teamwork?! Okay, Steven, are you ready to drill down into the planet to depths never before reached by your species to stop the Cluster before it forms and save your world?!"

Earthquakes begin rocking Beach City, indicating that the Cluster is only hours away from taking form. Steven and Peridot drill deep into the Earth where the Cluster is located but are unable to pierce the surface of it. As Peridot tries to destroy it, Steven sees faces and hears the voices of the gem shards comprising of that of the Cluster, and starts to talk to it as he falls unconscious. He convinces it not to form and, instead, to bond with itself. The Cluster begins to take form uncontrollably, however, and Steven attempts to bubble all the gem shards. He eventually wakes up and notices the gem shards inside the Cluster are bubbling each other. Steven finishes it off and merges all the small bubbles into one immense bubble, completely encasing the Cluster and preventing it from destroying the Earth.

"Steven Floats"
When Steven goes to the Big Donut, Sadie says that Steven missed all of the earthquakes, referencing those that were caused by the Cluster.

Trivia

 * The plan to hollow out the Earth was foreshadowed by Ronaldo's conspiracy theories in "Keep Beach City Weird".
 * Peridot refers to the Cluster as a "Gem geo-weapon". This is a play on the term "Bioweapon", Bio- referring to organic life while Geo- referring to rocks and minerals. This is appropriate, seeing as how Gems are a race of sentient gemstones and are thus inorganic.
 * In "It Could've Been Great", a map on the Moon Base reveals that the Cluster is currently located somewhere around Death Valley in Southern California/Southern Nevada.
 * Unlike Gem mutants, when the Cluster attempts to reform, its appendages are translucent.
 * Also, unlike Gem mutants, it possesses a rudimentary intelligence — likely because of a hive mind formed from the shattered intellect of the millions of shards that compose it.

Cultural References

 * The Cluster bears a great resemblance to Lavos, the main antagonist of the Chrono Trigger video game. They are both extraterrestrial creatures that will initiate an apocalyptic event if they are released/awakened unless a group of heroes stops them.

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Кластер Monstruos Gema