Utility company Ameren Missouri is investigating the cause of a small underground explosion in downtown St. Louis.

A transformer exploded around 6:30 p.m. Tuesday near the intersection of 10th Street and Olive, causing smoke to fill the street. No one was hurt. The fire was quickly extinguished.

Several downtown blocks were closed two weeks ago after an electrical fire began in a manhole in another area of downtown. Hot weather and aging equipment were blamed in that fire.