The Cluster

"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 currently incubating deep within Earth's core.

History
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.

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 "emergence" of The Cluster would be a catastrophic event that would shatter (kill) everyone.

In "When It Rains", Peridot says that after Homeworld no longer saw Earth as a viable colony to incubate gems, 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 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 comprised of millions of gem shards. She mentioned that it will be much bigger than Earth if it emerges and takes its physical form, and could destroy the planet once it does so.

Trivia

 * Peridot is utterly terrified of The Cluster, as she knows what it is and what it is capable of.
 * She often uses her knowledge of The Cluster as leverage to prevent the Crystal Gems from harming her, but it functions as a stalemate at best.
 * The plan to hollow out the Earth was foreshadowed by Ronaldo's conspiracy theories in Keep Beach City Weird.