CaCO3 is an important component of marine sediments and one of the major deep-carbon carriers at subduction zones. Some subducted CaCO3 can be dissolved in subduction fluids and recycled back to the surface via arc volcanoes degassing. At the same time, there still remain large amounts of CaCO3 and its reaction products, which could be further transported into Earth’s deep interior. These internal processes link atmosphere, hydrosphere and biosphere with the deep solid Earth, modifying the environments of our planet. In this review, we summarize current understanding from high pressure-temperature experiments and field petrological observations on the physical and chemical properties of CaCO3. In particular, the phase stability and reactions of CaCO3 largely control the migration and reservation of oxidized carbon in subducting slabs. Finally, we present several critical but unsolved questions on CaCO3 subducting in the deep mantle.