When I was much younger, my Father did this under our house. As tomh said, it was extremely labor intensive, and ended up being a 2 year project. My Dad even built his own dump trailer to haul the dirt. If it wasn't for all the relatives and lending hands from friends, it would have never been finished.
With the use of todays mini excavators, skid steer loaders, etc, the right contractor could make short work of the project. The cost would be very high, and you would probably never recoup the investment.