Geological conditions deep below Rüdersdorf are optimal for building caverns, because 150 million years ago there was a sea here and what remains of it today is a layer of salt rock between 600 and 2,500 metres under the surface.In February 2021, EWE erected a drilling rig on an existing borehole that leads down to the salt rock.A pipe, slightly wider than a ring binder, is pushed down through the borehole, down to where the test cavern will be created at a depth of 1,000 metres, and cemented into place.Water is pumped underground through the pipe, dissolving the salt and forming a 500-cubic metre cavity filled with salt water (called a “sole”), about the size of a swimming pool 25 by 10 metres across and 2 metres deep. This would make the test cavern only around one thousandth of the size of the natural gas cavern most recently built on the Rüdersdorf site.When the cavern is later filled with hydrogen, the salt water is removed and disposed of in a climate-friendly manner. A pipeline transports the sole to a brine operating station in Heckelberg, where it is piped into a natural underground salt-water reservoir.EWE built this station in the late 1990s when the first cavern was built in Rüdersdorf and will continue to operate for expansion measures such as this.