Advanced case of emphysematous pyelonephritis. The vast majority occur in those with diabetes mellitus and/or urolithiasis.
In this case it is present bilaterally and on the right has perforated the renal capsule to extend in the most profound fashion into the retroperitoneum. This is termed class 4: the highest class of the disease.
In fact, the gas serves to illustrate the anatomy of the retroperitoneal spaces and confines of Gerota's fascia.