Enormous cavern below major Toronto intersection is getting even bigger

Construction has reached a fever pitch on the downtown Toronto stretch of the future $27 billion Ontario Line subway.

The new transit line will relieve existing subway interchanges with a 15-station, 15.6-kilometre route between Exhibition Place and the shuttered Ontario Science Centre — including a six-kilometre underground section between Exhibition Place in the southwest and the Don River in the east 

The stations being constructed along this tunnelled section are among the most complex and expensive on the line, involving unique construction methods and intensive subterranean mining, all without disrupting the already-chaotic downtown traffic situation.

Metrolinx recently shared a fascinating update on the future King-Bathurst station, where crews have been working to excavate a cavernous void below the stop's namesake intersection since last year.

The vast hollowed-out space being mined below King and Bathurst, known as the north site cavern, recently crossed an important milestone when both ends of the cavern (known as the headwalls) were reached by crews. 

Headwalls mark the end-to-end limits of the station site, bookending the cavern. 

Since headwalls were reached, crews have continued to widen and deepen the cavern between — a 24-hour, seven-days-a-week process highlighted in striking images shared by the transit agency.

Some of the equipment being utilized for this process includes roadheaders, rockbolters and shotcrete machines.

A staggering quantity of rock and soil has been removed from the cavern, with approximately 21,000 cubic metres excavated to date.

The completed cavern will be wide and deep enough to accommodate the tunnel boring machines that will soon begin carving their way from a launch shaft near Exhibition Place.

As work progresses for the cavern at the north end of the site, crews continue excavation for what will eventually become the station's entrance and platform access areas, referred to by Metrolinx as the station shaft.

This area of the station is being excavated in cycles, and Metrolinx explains that the work involves "coordinated phases of rock breaking followed by installation of structural reinforcements, called struts and walers."

This dig-down has exceeded 25 metres below street level as of mid-2025, but there is still a long way to go before the excavation bottoms out at a final depth of approximately 40 metres.

In contrast to the 24-hour work cycles happening at the north excavation, work at the south side of the site is occurring from Monday to Friday, from 7:00 a.m. to 11:00 p.m., Saturdays from 8:00 a.m. to 5:00 p.m., and some Sundays as needed, from 8:00 a.m. to 5:00 p.m.