Why do we want to store carbon dioxide underground? Natural geological systems hold buoyant fluids such as oil for millions of years, so we have confidence that underground storage will work. What is a saline aquifer? A porous and permeable rock naturally filled with salty water What is a trap? A structure in the rock that the carbon dioxide gets trapped in Why do mudstones (or shales) make good seal rocks? The carbon dioxide will be trapped below them, effectively permanently Why do we need mechanisms to trap the carbon dioxide underground? Because carbon dioxide is buoyant, and will rise back to the surface unless something stops it.