When firefighters arrived about 8 a.m., Stafford said the breakers on Wabash Avenue had already tripped.

“That cut the power that was going to those transformers, which in turn put the fire out and caused the explosions to cease,” he said. The smoke eventually dissipated.

Authorities closed a major block of Peachtree Street while they investigated. Linden Avenue was also blocked between Courtland and West Peachtree streets.

Linden was reopened to traffic about 10:15 a.m. All but one southbound lane of Peachtree Street was reopened by 4 p.m., but authorities closed the road again on Thursday to continue repairs, according to the WSB 24-hour Traffic Center. Peachtree was shut down in both directions Thursday morning between Linden Avenue and Pine Street, but reopened all but one lane shortly after noon.

Major streets in Midtown were blocked due to an underground electrical explosion. JOHN SPINK / JSPINK@AJC.COM

AJC.com readers tweeted videos from the scene showing a series of quick, loud pops and flashes of light coming from grates under the sidewalk near the hospital. According to Georgia Power, those grates are intended to ventilate the underground area where the transformer is housed.

When things quieted down, thick plumes of smoke were seen rising several stories into the air over Peachtree Street. John Kraft, a spokesman for Georgia Power, said it appears the issue was isolated to a single transformer.

Initially, officials said two or three manholes were “blown off” from the power of the explosion.

“After further investigation and speaking with Georgia Power representatives, we can confirm that no manhole covers were ‘blown off’ as initial reports from 911 calls indicated,” Stafford said in a statement posted to Twitter about 12:40 p.m.

The hospital operated on backup power for a time, officials with Emory Healthcare told AJC.com. Power has since been restored to the hospital.

“There were no injuries and patient care continues at the hospital as normal,” spokeswoman Janet Christenbury said in a statement. “No evacuations were necessary, but some patients were moved within a care unit to ensure their safety.”

Atlanta fire crews perform a check after an underground explosion was reported near Emory University Midtown. JOHN SPINK / JSPINK@AJC.COM

Georgia Power crews are still on the scene investigating and making repairs.

— Staff writer Amanda C. Coyne contributed to this article.