In most cases it is replaced with water, either by design (when we pump it in) or by natural permeation from surrounding rocks (rain seeping through will replace it eventually). Sometimes, the surrounding earth structure is good enough to support voids in the structure.

In most cases it is replaced with water, either by design (when we pump it in) or by natural permeation from surrounding rocks (rain seeping through will replace it eventually). Sometimes, the surrounding earth structure is good enough to support voids in the structure.

From our human perspective on earths surface, as we all know, it feels like its still.  But we also know, if we aren't too ignorant or uneducated, that  its actually moving both around on its own axis and through space around the sun in its elliptical orbit at hundreds of thousands of miles per hour.  … Read more