In the 1968 novel "Stand on Zanzibar," author John Brunner projected that if the 7 billion people alive in the year 2010 were to stand shoulder to shoulder, they would cover the island of Zanzibar. According to Wikipedia, "Throughout the book, the image of the entire human race standing shoulder-to-shoulder on a small island is a metaphor for a crowded world where each person feels hemmed in by a prison made not of metal bars, but of other human beings."
This book I read 30 years ago came to mind as I was in the middle of eight weeks working in Manhattan. The island is only 1/20th the size of Zanzibar, so apparently it would be physically impossible for the entire population of Earth to be milling around in Times Square at all hours of the day and night. It just seemed that way.
Actually, the daytime population density of Manhattan (including commuters) is about 125,000 people per square mile. Each Manhattan resident and worker gets 220 square feet of land, in theory a luxurious 100 times more than the space required for standing and not being able to move an inch in any direction. (My hotel's elevators on Saturday mornings seemed to be an evil experiment to measure exactly how tightly humans can be packed.) On a cheery note, this land density doesn't take into account all floor space provided by skyscrapers that allow people to be stacked to the sky.
My next work assignment is to go stand on Dewey County, South Dakota, which is 100 miles from nowhere and has fewer than three people per square mile. Magnificent desolation, no packing or stacking required.
I have posted a bunch of images from my time in New York City and vicinity.