Caverns are large underground cavities that can be constructed in rock by mining or can be created in salt formations by leaching with fresh water.

In salt caverns, large quantities of gas can be stored under pressures of 250 bar and more. Natural gas storage in salt caverns has been established in Germany since the 1960s. In addition to natural gas, other substances can also be stored in salt caverns. A large part of Germany's crude oil reserves has been stored in salt caverns since the oil crises in the early 1970s.

Such salt caverns are therefore also suitable for storing electricity as compressed air storage. These caverns are safe, the technology is mature and the potential for building more caverns is huge.