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

WorkSafe BC is now leading the investigation into an underground explosion in downtown Vancouver that sent three BC Hydro workers to hospital.

One worker remains in hospital with serious injuries, while two other workers have been released.

The three workers were doing hydro vault maintenance at the intersection of W. Georgia and Seymour streets when a manhole blew out at about 11 a.m. PT.

All three were taken to hospital, with one worker suffering serious burns.

"In these vaults you can have a significant amount of power," said WorkSafeBC spokesman Scott McCloy.

"There were literally thousands of volts of power going through these various switches they were doing routine maintenance on ... and somehow there was an explosion. So what we need to do is determine why this explosion happened, because it shouldn't have."

McCloy said all the documents pertaining to the job have been seized.

He said witness statements from other BC Hydro employees and bystanders in the area at the time of the accident are also being collected.

"People go to work every day thinking they'll be going home at night and these three unfortunate workers from BC Hydro, they were injured when they shouldn't have been," McCloy said.