Massive cavern forming below major Toronto intersection looks like a mine

A large void is being carved out below a major Toronto intersection traversed by thousands every day, and few people passing overhead have any idea that a huge cave lurks below as part of the push to build the new $27 billion Ontario Line.

When it opens to the public in 2031, the new 15-station, 15.6-kilometre transit line will relieve existing subway interchanges by providing a route between Exhibition Place and the shuttered Ontario Science Centre.

That route includes a tunnelled stretch through the heart of downtown Toronto, requiring deep underground excavations at station sites in preparation for the arrival of tunnel boring machines.

The future Ontario Line Queen-Spadina subway station will be one of the stations requiring the excavation of "caverns" deep below street-level, and Metrolinx has provided an update on the major operation hidden out of sight underground.

The transit agency shared new photos celebrating "a big month underground" in February, noting that "progress is moving full steam ahead," including tasks like "lowering specialized, heavy-duty equipment into the shaft to making headway on station cavern excavation."

February was a big month underground! From lowering specialized, heavy-duty equipment into the shaft to making headway on station cavern excavation, progress is moving full steam ahead. Stay tuned for more updates as we carve out the tunnel for the future Queen-Spadina station. pic.twitter.com/LxFF6aaqXp

— Ontario Line (@OntarioLine) April 21, 2025

Metrolinx announced the start of excavation at the Queen-Spadina site in early February, sharing that "cavern excavation is underway and will continue at both north and south sites, into early 2026."

"Cavern excavation involves crews connecting the newly completed station shaft to the future station platform area and excavate[ing] in a southwest direction away from the northeast corner of Queen Street and Spadina Avenue," read a release issued by the transit agency earlier this year.

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

This task involves specialized excavation machinery called a roadheader and a technique called sequential excavation to shave away the walls of these subterranean spaces, destined to host the new station's subway platform. Excavated material is then hauled out from the dig site and transported away in dump trucks.

Crews are working in rotating shifts, 24 hours a day and six days a week, with occasional Sunday shifts as needed.

Diagram of the cavern being excavated below the Queen and Spadina intersection. 

A similar void is being carved below the intersection of King and Bathurst, where crews are carving out space for the future King-Bathurst station.

Mining operations for King-Bathurst commenced last December and are expected to continue until late 2025.