Why sequester CO₂ underground?

The sequestration process is used because it replicates the natural processes that have held water, oil, and gas underground for millions of years. When sequestration sites are properly selected and used, CO₂ is expected to remain stored permanently without escaping into the atmosphere. When the CO₂ is injected into the sequestration formation, there are four types of trapping that occur to keep CO₂ permanently underground due to solubility and structural trapping (see question 14 below for information on the four types of trapping). The CO₂ is eventually converted to rock over thousands of years.