An "underground repository" refers to a storage facility that is located below the ground. It is often used to keep important or dangerous materials, like radioactive waste, securely away from people and the environment. Full definition
No country with nuclear power has a viable underground repository for waste, and proposed sites in France face public opposition, despite more widespread support for nuclear power. (scientificamerican.com)
They exposed the forsterite to water - free supercritical carbon dioxide at conditions similar to those found in underground repositories targeted as storage sites: 50 degrees Celsius and 180 atmospheres of pressure. (pnnl.gov)
However, Brent says that suitable underground repositories are hard to find and that carbon dioxide may escape back into the atmosphere even after being stored. (newscientist.com)