Police tape surrounds a sidewalk manhole at the corner of Seymour and W. Georgia after an explosion Wednesday. (CBC)

Two BC Hydro workers suffered burns and one other had undetermined injuries after an underground explosion in downtown Vancouver Wednesday morning.

The workers were doing hydro vault maintenance at the intersection of West Georgia Street and Seymour Street when a manhole blew out at about 11 a.m. PT, sending black smoke billowing into the air.

"The crews are reporting some second-degree, possibly third-degree burns ... to some of the workers," said Vancouver assistant fire chief Wade Pierlot.

"When I saw the gentleman ... come out of the manhole, he fell right over," said Suki Sadhre, owner of Van City Sports. "He had a tough time breathing. And then the next gentleman that came was covered in black and I thought, 'Oh, my God.'"

The incident is under investigation by WorksafeBC.

"We're not certain what happened but we know three were injured," said BC Hydro spokeswoman Susan Danard. "It's of grave concern for us. Our focus is public safety and employee safety."

Danard said the explosion did not cause any power outages.

A BC Hydro internal investigation is also underway.