The Cluster (Canon)

"But these were just prototypes for the final product, a singular giant artificial fusion, comprised of millions of Gem shards... The Cluster."

The "Cluster" is a massive Cluster Gem that was first mentioned in "Jail Break" and first seen in "When It Rains". Composed of millions of gem shards, it is currently incubating deep within Earth's mantle. In its emerged form, the Cluster will be larger than the Earth.

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

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

"When It Rains"
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 will 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"
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 to try and reach the Cluster before it finishes its incubation period.

"It Could've Been Great"
The Cluster was revealed to have been inserted in the Beta Kindergarten into the mantle approximately 2,500 units down relative to Greg's Storage Shed.

"Message Received"
After taking the Diamond Communicator, Peridot argues in favor of the plan to terminate it to Yellow Diamond. Yellow Diamond replies she couldn't care less that it will destroy the Earth, and orders for Peridot to let the Cluster continue to grow.

Trivia

 * Peridot is utterly terrified of the Cluster, as she knows what it is and what it is capable of. Before her truce with the Crystal Gems, she often used her knowledge of the Cluster as leverage to prevent them from harming her, but it functioned as a stalemate at best.
 * 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 southern California/southern Nevada.

Cultural References

 * The Cluster bears a great resemblance to Lavos, the main antagonist of the Chrono Trigger video game. Both of them are extraterrestrial creatures that absorbs the energy from their host planet while being buried beneath the surface. They will both initiate an apocalyptic event if they are released/awakened.