The authors describe how water seeps into the cavern via fractures when it rains, running across the floor where it is held in depressions. The biggest and most elevated of these basins shows ...

Engineers can drill straight down and inject water to dissolve that salt, creating huge empty caverns. The brine can then be pumped out and gas can be pumped in to be stored for when it is needed.

Keep in mind the camera added light for the picture; much of the cavern was darker than it looks in the pictures. The green you see is in the picture below is actually water collected in the cavern.