We obviously won't know anything with certainty until Thursday, at least, but this just occurred to me, so I thought I'd share.
In the promo for "When it Rains," Peridot is shown to fear a rainstorm, stating "It's happening!" "It" is likely referring to the emergence of the Cluster, which suggests a storm is involved in the latter. Now, where have we heard about a storm before?
The Geode. Pearl stated that it contains a "synthetic storm"- but what if, unbeknownst to the Crystal Gems, the Cluster is incubating inside/beneath it? This would also account for why it had started cracking in the first place- the Cluster wasn't fully mature yet, but it had nonetheless started hatching. When it hatches, the Geode will break, releasing both the storm and the Cluster.
The Cluster is inside the earth's core so it cannot be in the Geode, the Geode is just a weapon that contains a very strong electrical storm, and Peridot saying its happening it just surprise from precipitation on earth.