Groundhogs are excellent burrowers, using burrows for sleeping, rearing young, and hibernating. W. J. Schoonmaker excavated 11 dens finding the earth removed from these averaged per den. The longest burrow was 24 feet plus 2 short side galleries. The amount of soil taken from this den was and it weighed . The average weight of the earth taken from all eleven dens was . Though groundhogs are the most solitary of the marmots, several individuals may occupy the same burrow. Groundhog burrows usually have two to five entrances, providing groundhogs their primary means of escape from predators. Burrows can pose a serious threat to agricultural and residential development by damaging farm machinery and even undermining building foundations. In a June 7, 2009, Humane Society of the United States article, "How to Humanely Chuck a Woodchuck Out of Your Yard", John Griffin, director of Humane Wildlife Services, stated you would have to have a lot of woodchucks working over a lot of years to create tunnel systems that would pose any risk to a structure.