Fire officials said construction workers struck a gas line near State and Randolph streets around 12:15 p.m. Tuesday.
The pressure caused a gas cap to shoot up about a block away, near State and Madison streets. The small explosion did not cause any injuries.
A Level 1 hazardous materials response was called, which was later expanded to a Level 2 hazmat. State Street was closed to vehicles and pedestrians between Wacker Drive and Madison to give crews room to work.
"We're just making sure the old retired main does not have any gas left in it. It's going good right now. We've also checked all the areas around it - the subway, the basements in surrounding buildings and lobbies in surrounding buildings - and we have no gas readings at all at this time," said William Vogt, Chicago Fire Department.
State Street was closed from Wacker to Madison until the remaining gas was burned off.
The #6, #10, #14, #29, #36, #62, #146, #147 and #151 buses that usually run on State Street were temporarily rerouted, the CTA said.
Red Line service was temporarily disrupted, but normal service has resumed.