Most people in Chicago had no idea that a huge network of tunnels ran beneath their city. The tunnels were built in the early 1900s to carry telephone wires and deliver coal. Over the years, people forgot about them. On April 13, 1992, workers driving pilings near the Chicago River accidentally cracked a tunnel wall. Cold river water began seeping through the crack. City leaders knew about the leak but thought it was small. They were wrong. The crack grew quickly, and millions of gallons of water surged into the tunnel system. The water spread through forty miles of tunnels under downtown Chicago. It flooded the basements of office buildings, stores, and even City Hall. Electricity was shut off to prevent fires. Over 250,000 workers were sent home. The subway system filled with water and stopped running. Businesses lost millions of dollars in damaged merchandise and equipment. Pumping crews worked around the clock. It took weeks to drain the tunnels completely. The flood caused about two billion dollars in damage. Engineers later sealed the crack and inspected the entire tunnel system. The Great Chicago Flood taught a lesson: cities must keep track of the hidden structures beneath them.