A manhole fire caused a loud explosion sound near Union Square Friday morning, the FDNY said. 

A call came in shortly before 8 a.m. about a fire in a manhole at Broadway and 12th Street, a spokesman said. The FDNY tweeted that multiple manholes were on fire, but did not have an exact number. 

advertisement | advertise on newsday

People nearby tweeted that they heard a loud explosion sound. 

“Giant explosion behind me on Broadway as I was walking to work,” one user wrote.

The cause of the fire in under investigation.