In the early 1800's water mains were made from wood, like the section seen here. When the fire department arrived on the scene they would dig down to the water main and chop a hole in order to access the water. Once the fire was struck out the hole was filled with a “fire plug” and the location was marked for future access. Upon returning to the location the pick end of the fire axe was used to pry the plug out of the hole.
The old wooden mains were replaced by a piped water main system with fire hydrants placed roughly 300' apart. The hydrants have 2 x 4 ½” outlets. The first engine company to access the hydrant will connect the engine to one side of the hydrant and place a gate valve on the second unused outlet for future use. Contrary to what Hollywood would have you believe those scenes where a car crashes into a fire hydrant and causes a fountain of water are untrue. Chicago fire hydrants have shut off valves roughly 7' below the hydrants, so if the hydrant gets knocked off, the operating stem that connects the wrench nut to the seating valve snaps off and the valve remains closed below the ground.