This huge cave hidden under Toronto will soon be a busy subway station

A future Toronto subway station is being carved out below a busy downtown intersection, just one of the so-called "caverns" laying the literal groundwork for the forthcoming $27 billion Ontario Line.

At the core of the 15-station, 15.6-kilometre route between Exhibition Place and the now-defunct Ontario Science Centre, a six-kilometre tunnelled section will carry trains in underground tunnels spanning from Liberty Village to the Don River.

But before tunnel boring machines can chew these tunnels, the station sites must be primed with excavation — a complex commission for construction crews to carry out in the constantly congested core.

The ongoing excavation at the future Queen-Spadina Ontario Line station bears striking similarities to the operation at the nearby King-Bathurst stop, with vast subterranean voids now forming what will eventually be the stations' platforms and tunnels.

At the Queen-Spadina site, Metrolinx has shared a detailed construction update that offers a behind-the-scenes glimpse at the current progress, including a look at the impressive "station cavern" forming deep below the intersection.

Work at Queen-Spadina has been ongoing since 2023 with the gutting of structures that will live on at the station site. However, there's a whole lot of work to do before these holes in the ground are transformed into transit stations.

The future station work area is divided into a north site and a south site, and crews are in the process of excavating the area where the future station platform and tunnel will be situated between these two locations.

Similar to the cavern taking shape nearby at King and Bathurst, the Queen-Spadina cavern recently completed end-to-end excavation of its cavern, which is now being enlarged ahead of the soon-to-launch tunnel boring machines.

This milestone saw crews reach station headwalls, which act as bookends to the cavern and mark the extent of the station excavation ahead of the tunnel boring machines several months down the road.

With the full length of the station now partially excavated, crews are working to widen and deepen this cavern to a sufficient size to accommodate the tunnel boring machines, which will break through the south headwall before passing through the station to begin chewing through the north headwall.

Crews continue to widen and deepen the cavern, to accommodate the full width and depth needed for tunnel boring machines, as well as the future station platform and tracks.

Crews are using specialized machinery to remove soil and rock in a process known as sequential excavation. Excavated material, known as spoils, are removed via the future north station site.

Excavation work is being carried out on a 24-hour schedule, seven days a week, with an ambitious goal to have this phase of construction completed later this year.

Over at the future south station site, crews continue to remove soil and rock to form the underground void that will form the southern extent of the stop.

According to Metrolinx, 50 per cent of this excavation work has been completed to date.

The Ontario Line is expected to welcome its first riders in 2031.