Once the boys get to the rail bed and they find that they have to dig really far down to get to the pipes, they settle in for a long bit of work.  As they dig, they get filthier and filthier and they begin to resemble what they think of as natives, browned by the dirt and the sun, getting crusty and in need of haircuts, etc.  And as they become more and more like these natives, they actually finish diggin and they are laying around worshiping the finally exposed pipe like "the mud men of borneo worshiping a fallen idol."  Of course then things really go south when Homer gets hurt pretty badly.