Here's why:

* Vast Storage: Rocks, particularly sedimentary rocks like limestone, contain the vast majority of Earth's carbon, estimated to be over 66 million gigatons.

* Geological Time Scales: This carbon has been locked away for millions of years through geological processes.

* Other Reservoirs: While rocks hold the most, other reservoirs like the ocean, the atmosphere, and fossil fuels contain significant amounts of carbon, but they are dwarfed by the amount stored in rocks.

Let me know if you have any other questions!