Caused by the flow of sand and water ingress chandin surface to prevent access to stored in the cavity floor rocks with pore spent dozens of years, or hundreds of thousands in deep level from 10 meters to hundreds of meters.
Caused by the percolation of surface water, soil and sand into shale trapped in the hollow porous spent dozens, hundreds or thousands of years in depth from 10 meters to several hundred meters.
Caused by seepage through soil and sand surface water into the hold in the hollow rock porous take dozens, hundreds or thousands of years at a deeper level, since 10 meters to hundreds of meters.