It was called Kowloon Walled City.
With a population of 33,000 squeezed into the tiny lot, the city was 119 times denser at its peak than present-day New York City. Although it faced high levels of crime and poor sanitation, the city was also impressively self-sustainable - until its demolition in 1993.
In the late '80s, Canadian photographer Greg Girard found his way into the windowless world.
He shared some photos and thoughts on his time in Kowloon Walled City with us. You can check out the rest, along with essays and work from photographer Ian Lambot, in the recently released book "City of Darkness: Revisited."