A blog in the Fargo-Moorhead area considers how many sandbags are 3.5 million sandbags?
He/she determines that it could build a wall that would encircle the Empire State Building.
I didn't realize, by the way, until last week that homeowners in the Moorhead area were going to have to buy the sandbags (41 cents each) and the sand to go in it. But in building a dike, it was decided that it wouldn't be wise to give people an incentive not to build them as strong as possible.
But in the past, one resident told me, the residents would buy the sand and the bags, and then when the flood was over, they'd dump the sand in the street and the city would come by and pick it up. Through that method, the city got free sand.