Sunil,
That bunker is a tornado shelter. That part of the U.S. is in the area called "tornado alley." It's a steel reinforced concrete bunker buried six feet into the ground with heavy steel doors. It was there before he added onto his house. Almost everyone has a tornado shelter, just in case.
...by the way, my Pond Boss was in the P.O. box yesterday. (Yes, we mail one to me, just so I'll know how long it takes to get here.)
