WFR responded to a strong gas smell in the area of 1315 5th Ave. SE. at 02:26AM. Engine 4 crew on scene found a strong gas smell outside.  Municipal Utilities and Engine 4 crews investigated all gas valves on buildings in the area. The gas leak was determined to be underground near a new sign that had been recently installed. Engine 4 and Utilities crews investigated inside the adjacent buildings for gas levels.  All buildings are clear of gas.  5th Ave is closed between 13th St. and 14th St. SE as utility crews are digging to plug the 4" gas line that is leaking.  Engine 4 was on standby and staged at the corner of 5th Ave. and 14th St. SE,  Assisting with monitoring 3 buildings downwind from the leak.  After the lines were capped engine 4 returned to station at 0936AM.  Utilities remaining on scene to dig and remove the damaged section. The road will remain closed until utilities are finished.