The presence of a mysterious gasoline or oil-like substance inside the subway tunnel near College station closed the Yonge line between Bloor-Yonge and Union at the height of rush hour this morning.

The TTC called the incident, which began around 11:00 p.m. last night, an "environmental spill." CEO Andy Byford said there were concerns a spark within the tunnel from a passing train could ignite the mystery substance. The line was closed as a safety precaution and so far hasn't re-opened.

Earlier this morning, TTC spokesman Brad Ross tweeted pictures of the tunnel walls and floor coated in a yellow-tinged slimy substance. Multiple agencies, including Enbridge, Toronto Hydro, Toronto Fire, and the City of Toronto, are investigating the source and nature of the leak.

North of College, track-level. Leak is oil-like, unsafe to operate. Planning to grout tunnel joints. Update to follow pic.twitter.com/zC6pOuytZk

— Brad Ross (@bradTTC) March 24, 2015

TTC work crews are plugging the joints in the tunnel wall to stop the leak, but there are no guarantees service will be restored before the afternoon rush. The repair and clean-up will take two or three hours to complete.

Speaking to reporters near College station, Byford said he believed the substance was diesel or kerosene, possibly leaking from an underground hotel storage tank cracked by the Spring thaw. "It's actually coming through pretty rapidly," he said. "We could not take any chances with this liquid."

"Having been down onto the track, seeing how much of this liquid is coming in and smelled it, we've done exactly the right thing in putting safety first ... customers I've spoken to this morning have had no qualms whatsoever with the action we've taken. You could not possibly risk running an electric train through a confined tunnel where there's a risk of combustion or a flashover."

"This is not generated by the TTC," Byford said.

Despite 70 shuttle buses operating between Rosedale and St. Andrew stations and the TTC urging riders onto alternative routes, the situation bordered on the chaotic all morning. With the busiest section of the Yonge line closed at peak hour, tweets about jam-packed stations, gigantic line ups for shuttle buses, and all-round misery flooded Twitter.

Here's a look at how the situation unfolded.

UPDATE 11:40:

#TorontoFire subway tunnel leak test found small amount of hydrocarbon brought in from groundwater - no explosion hazard

— Toronto Fire Service (@Toronto_Fire) March 24, 2015

Chris Bateman is a staff writer at blogTO. Follow him on Twitter at @chrisbateman.

Image: @scumm_boy/Twitter