A section of King Street will remain closed in the Financial District today after a fire in an underground hydro vault shut down the busy downtown street on Monday evening.

Brown smoke could be seen billowing from a sidewalk grate outside Scotia Plaza on King Street West shortly after 5 p.m. The smoke was accompanied by several loud pops that sounded like a series of explosions.

It was later discovered that the smoke was caused by a fire in a hydro vault, which contained high-voltage transformers.

The incident prompted emergency responders to evacuate the Royal Bank of Canada office building at 20 King Street West and part of the underground PATH system.

The smoke, according to Toronto Fire Capt. Adrian Ratushniak, contained carcinogens and members of the public were advised not to breathe it in.

King Station was shut down on the Yonge-University-Spadina Line but has since reopened.

King Street West is still closed between Yonge and Bay streets and officials say it will likely be blocked for most of the day.

Firefighters called back to vault:

Hydro crews had intended to work through the night to try to make repairs but firefighters were called back to the vault early Tuesday morning after more smoke started to emit from the vault.

"It took us until about midnight to get into the site because Toronto Fire had to clear the area and get rid of the smoke and everything like that. We had to pump water out. And then while we were in there, it started to fill up with smoke again because there was something still smoldering," Brian Buchan, a spokesperson for Toronto Hydro, told CP24.

Toronto Fire Platoon Chief Kevin Shaw said a fire in a conduit in the vault was to blame for the delay in repairs.

According to Shaw, crews attempted to extinguish the small fire in the conduit by putting a small amount of water on it but Toronto Hydro crews could not get back into the vault until late Tuesday morning.

Toronto Fire Platoon Chief Kevin Aucoin said Toronto Hydro crews are going into the vault to try to remove the damaged transformer and replace it.

"Right now (Toronto Hydro) is going to be coming in with some heavy machinery. They are going to be removing some concrete slabs where the damaged transformer is. They are going to have to reroute some power supply to another transformer before they actually remove the damaged one and it is going to be a long process," he noted.

"We have our crews on scene. We have our confined space rescue crews here in case anything were to go wrong. We’re kind of dealing with the fact that… the wires running within the conduit to the transformer room were still smoldering so we are keeping crews on scene while they do their work."

A new transformer module was lowered into the vault on Tuesday afternoon.

King Street to remain closed Tuesday:

Officials initially said that King Street could reopen at some point on Tuesday morning but Aucoin told CP24 that it will likely be closed for the day.

"I’m told by (Toronto Hydro) they are going to need the better part of the day because we are going to have heavy machinery coming in and out of the area," he said.

"So King Street will be shut down today."

The exact cause of the fire has not yet been determined.

"We had the torrential downpours yesterday so it is possible that the water level rose to the point where it didn’t drain out and it overwhelmed the system and then therefore fried everything," Buchan said, adding that the transformer is not a particularly old piece of equipment.

The transformer was inspected this past fall, he noted.

"Sometimes when you have a severe weather event, it may overwhelm systems just like it might flood your basement," Buchan said. "In this case maybe it flooded the vault. We don’t know yet."

The Royal Bank office tower remains closed today but surrounding buildings are open. Toronto Hydro said the Royal Bank tower has not had hydro service since Monday night.