In 2004, photographer Gina LeVay began documenting the proudly self-proclaimed "sandhogs," or underground construction workers, who for generations have built and maintained, among other things, the mind-bendingly huge tunnel system that supplies New York City with more than a billion gallons of safe—and quite tasty—water from upstate reservoirs every single day. LeVay spoke with LIFE.com about the project, the sandhogs she met and became friends with, and the humbling thrill of chronicling the vast, intricate world that lies beneath.