The Black Rock is a mysterious asteroid which hovers around the planet around every five years, before disappearing after one week. In the events of Sonic Robo Blast 2, Dr. Eggman catches wind of the Black Rock and harnesses its power for his nefarious needs. The discovery of the Black Rock is what sets the story in motion, as Dr. Eggman uses its power to destroy Greenflower Mountain and alert Sonic of his return.
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The Black Rock remains unseen for the rest of SRB2, but reappears as the foundation for the game's final two zones. Egg Rock Zone, the penultimate zone, shows that Dr. Eggman has managed to encapsulate and fortify the Black Rock. The player travels along the surface of the asteroid in the many outerspace sections of the acts. Black Core Zone, the final zone, has the player journey deeper into the Black Rock and discover its core in Black Core Zone Act 3. The core of the Black Rock is shown to be a glowing green gem-like structure, similar in appearance to the Master Emerald.
After defeating Brak Eggman with all seven Chaos Emeralds, the Black Rock is freed from Dr. Eggman's confinement by the emeralds and disappears into space. The way in which the Chaos Emeralds resonate with the Black Rock in both the ending and after-credits scene suggests a deeper connection between the emeralds and the mysterious asteroid.