So, I realised; in the episode Mirror Gem, Connie and Steven are talking. Connie tells Steven about how "school's out", and introduces him to the concept of school. He then has Pearl give him a class, but it then ends with Pearl showing him The Mirror. Steven has fun with it, realises it's sentient, and releases Lapis from the mirror. She then attempts to flee the Earth and return to Homeworld, succesfully doing so in the following episode, after Steven heals her cracked gem. Lots of episodes later, in The Return, Lapis returns, with Jasper serving as her escort and Peridot as her informant. They take the Crystal Gems into the ship on course for Homeworld, Garnet then defeats Jasper and crashes the ship. Lapis and Jasper fuse, Malachite is formed and then held captive at the bottom of the ocean. Peridot escapes, is captured, becomes smol, etc etc. She tells the Crystal Gems about The Cluster, and they start building the drill; then, Malachite returns, fights with Alexandrite and is unfused. While this happened, Steven and Peridot drill down to the Cluster with the purpose of destroying the Cluster, but they end up bubbling it instead, thus preventing the Earth from being destroyed, and preventing a colossal Gem Weapon from falling into the hands of Yellow Diamond.
TL;DR: Thanks to Connie telling Steven about school, the Earth was saved from destruction.