The answer is: A stone like a great millstone

Rev 18 vs 21

In Revelation 18:21, when the mighty angel casts a stone like a great millstone into the sea, it's more than just a visual spectacle. This dramatic act signals the total and irrevocable downfall of Babylon—a symbol of oppressing powers and corruption. The millstone reflects the weight of judgment, illustrating how unbearable the wickedness of Babylon has become. Like a heavy stone sinking to the depths of the sea, so too is Babylon destined to be erased from history.

Interestingly, millstones were essential in ancient food production. They were used to grind grain into flour, which makes this metaphor even richer. Just as a millstone grinds grain into edible essentials, Babylon was known for grinding the lifeblood out of its people, trapping them in cycles of greed and excess. The image of it being submerged conveys how deeply entrenched such oppressive systems can seem, yet also declares that they are no match for divine justice.

Biblical echoes abound throughout Revelation. The idea of a great weighty object thrown into the sea resonates with the story of the Pharaoh's armies drowning in the Red Sea during the Exodus, marking a moment of liberation. Also, recall Jesus’ words about a millstone in Matthew 18:6, warning of the severity of leading others astray. Ultimately, Revelation provides a hopeful vision: the fall of oppressive systems makes way for a restoration that offers freedom, justice, and a future filled with promise for those marginalized and oppressed.