They used a machine that sorta looks like a lawn-mower to go over all the gravel and compact it. It is pretty heavy with a flat bottom and just shakes a lot. The walls are set on a bed of clean, crushed stone instead of a footer to allow for better water drainage. Because of our house site, it was still best for me to have a footer on the front wall, and so there is - and I mean it's a FOOTER! It goes down 6 feet in the ground to solid rock and is filled completely with concrete, and then piled up about 3 feet above ground. It ended up being 120 yards of concrete.