After The Great Fire of Gotham in 1875, the city planners decided to regrade the street level of the city by a full two stories.
For the regrade, the streets were lined with concrete walls that formed narrow alleyways between the walls and the buildings on both sides of the street, with a wide "alley" where the street was. Brick archways were constructed next to the road surface, above the submerged sidewalks. Skylights with small panes of clear glass (which later became amethyst-colored because of manganese in the glass) were installed, creating the area now called the Gotham Underground.
In 1907 the city condemned the Underground for fear of bubonic plague, leaving (the now) basements to deteriorate becoming flophouses for the homeless, gambling halls, speakeasies, and opium dens. To this day, many parts remain closed off and condemned, a perfect haven for the darker and most depraved of Gotham City’s criminal element.