Janeth Hernandez drops a two-toned weighted disc into the chilly water at Lake Mead. She plays out a rope attached to the disc, which is marked with a black line every meter. Her classmates peer overboard down into the depths of the great man-made lake. Janeth counts out as each black mark sinks past the surface. "Ten meters. Eleven, 12..."

Her classmates call back: "I can still see it" and "Me too." At 17 meters, it finally becomes impossible to discern the disk.