Really. You could put the entire population of the Earth in a space one mile wide, one mile deep, and one mile high (one cubic mile). We'd all fit in one small corner of the Grand Canyon, which would take about 2500 cubic miles of dirt (or people) to fill.
How do we know? One cubic mile is 147,197,952,000 cubic feet. If, on average, one human being takes up a space 2 feet by 2 feet by 6 feet it means we occupy 24 cubic feet per person. Sure, kids will have extra room and some of us will be a bit squished (or a lot if you're a flabby American); but on average that's about right.
So divide 147,197,952,000 by 24 and you get 6,133,248,000, which means that in the year 2000 when the world population reached 6 billion we'd all have fit in a cubic mile, with room left over for another 133 million folks. The population now has reached over 6.7 billion so today there'd be a little overflow.
But now consider this. If we all stood side-by-side, shoulder to shoulder (and use the same 2 foot width per person), all 6.7 billion of us could make a line around the Earth over 102 times!
Our brains are pretty well equipped to deal with liner measures (rows of corn, lines of wildebeest), but we're easily befuddled by cubic measures. We intuitively know that a line at Starbucks with 6.7 billion people in it would be a long one. But it's hard to imagine that we could pile all those people on Chicago's O'Hare Airport and the pile would only be 500 feet deep.