Construction is now in full swing for the $27 billion Ontario Line, and Metrolinx has declared 2025 "the year of digging" for the 15.6-kilometre subway line through the heart of Toronto.

The transit agency shared a general update on the project this week, shedding light on the various excavation methods and preparation for tunnelling operations that have begun to shape the future 15-stop transit link connecting Exhibition Place with the former Ontario Science Centre via downtown Toronto.

Here's everything that's going on with the big dig to form the city's next major subway line.

Tunnel boring machines will make the trip to Toronto

The Ontario Line's tunnelled section from Exhibition Place to west of the Don River will be carved out by a pair of tunnel boring machines (TBMs) that are currently undergoing testing at their manufacturing point in Germany.

After being disassembled and prepared for shipping, these twin TBMs will begin the long trans-Atlantic voyage from Germany to Toronto, where they will eventually arrive at the launch shaft and future tunnel portal currently being excavated east of Exhibition Station.

From here, the TBMs will be reassembled and lowered 16 metres deep into the launch shaft to begin the tunnelling stage, where they will chew through rock and soil at depths of up to 40 metres below street level.

Giant pits are forming

Before the tunnels can begin to carve out this six-kilometre underground section through downtown, crews must first dig out the underground station locations. 

Multiple excavation methods are being used to prep Ontario Line stations ahead of the TBMs.

Anyone who has passed by Moss Park in recent months may have noticed a large void forming along the park's southern edge, where crews are using the cut-and-cover excavation method to prepare the future station at this site with a pit that will extend as deep as 40 metres below ground.

This relatively simple method sees a large pit dug and then covered up, similar to how Toronto's Yonge Line was constructed in the 1950s as a trench that was then (mostly) covered up.

This same method is being used at the future Queen Station site, albeit with far more disruption. Due to existing infrastructure above and below street-level, the cut-and-cover excavation for Queen is taking place in the currently-closed roadway.

Corktown is the third station along this stretch of tunnelled track that will be excavated using the cut-and-cover method.

Caverns deep below busy intersections

The other three sites on the tunnelled section, King-Bathurst, Queen-Spadina and Osgoode, are being excavated with the help of large machines called roadheaders.

These machines, typically used in the mining industry, are in the process of carving away paths through hard rock that will eventually link subway tunnels to underground "caverns" that will one day become busy subway stations.

These caverns won't be constrained subterranean spaces by any means, and will be more on the side of cavernous.

The cavern at King-Bathurst Station will be approximately 135 metres long, or just shy of the length of two Boeing 747 jets parked end-to-end. This space will also soar 17 metres high — equivalent to roughly a five-storey building — and with a width of 23 metres, which is comparable to a tennis court.

Metrolinx anticipates the Ontario Line will be complete by 2031.