Nonetheless, polymorphism alters carbonate compounds' stability at different depths within the Earth. To illustrate, laboratory simulations and density functional theory calculations suggest that tetrahedrally-coordinated carbonates are most stable at depths approaching the core–mantle boundary.[13][2] A 2015 study indicates that the lower mantle's high pressures cause carbon bonds to transition from sp2 to sp3 hybridised orbitals, resulting in carbon tetrahedrally bonding to oxygen.[14] CO3 trigonal groups cannot form polymerisable networks, while tetrahedral CO4 can, signifying an increase in carbon's coordination number, and therefore drastic changes in carbonate compounds' properties in the lower mantle. As an example, preliminary theoretical studies suggest that high pressures cause carbonate melt viscosity to increase; the melts' lower mobility as a result of the property changes described is evidence for large deposits of carbon deep into the mantle.[15]