The new Ontario Line subway is checking off milestone after milestone in an uncharacteristically speedy fashion compared to the typically glacial pace of transit construction in Toronto.

Expected to open by 2031, Toronto's next subway line will span 15.6 kilometres between Exhibition Place and the shuttered Ontario Science Centre, and will relieve overcrowding at key interchange stations like the dangerously over-capacity Bloor-Yonge station.

And it's getting a bit closer to that point every day.

A significant milestone was recently reached in the completion of a launch shaft for the line's tunnel boring machines (TBMs), and stations along the line's tunnelled portion through downtown Toronto are now being prepped for the arrival of these earth-chewing mega-machines.

The future Ontario Line King-Bathurst subway station has also crossed its own big milestone in preparation for the TBMs' arrival with the completion of vertical excavation for the station's north shaft.

Metrolinx issued a recent construction notice explaining what is next for the station, stating that "the next phase of excavation will begin this December 2024 and continue into late 2025."

During this phase of digging, crews will form a large void that Metrolinx refers to as a "cavern," extending southwest from the newly excavated shaft at the northeast corner of King Street and Bathurst Street. 

According to Metrolinx, "this cavern also creates space for the future tunnel boring machines to pass through as they create the Ontario Line subway tunnels through downtown Toronto."

This task will be carried out with the help of specialized machinery that will be lowered into the shaft piece-by-piece and then assembled below ground. One piece of machinery known as a roadheader will shave away at the rock and soil using a specialized rotating attachment.

Using this equipment, crews will use a mining technique called sequential excavation to form this sizeable underground chamber in a complicated operation that will occur unseen below the busy intersection.

But there's a lot more than digging required behind the scenes to form this large cavern that Metrolinx calls "essential" in creating space for the station's train platform.

The removed material, known as rock spoil,s will be raised from the station's shaft and trucked off-site.

Excavation at King-Bathurst station is happening out of sight of passersby, with a large temporary structure blocking off the dig site from view and, perhaps more importantly, earshot, which Metrolinx refers to as an "acoustic shelter." 

Mining for this cavern will be a 24-hour operation seven days a week using rotating crews to keep the task moving steadily and ensure worker safety.

These constant work hours can be frustrating for area residents, though Metrolinx assures the public that it has measures in place to mitigate air quality, noise, and vibration — the shelter being the most apparent evidence of these efforts.