1. A terror hideout

Ron Wretham, a 32-year Toronto police veteran, can’t get over the tunnel’s proximity to a Pan-Am Games venue. While Deputy Police Chief Mark Saunders told reporters Tuesday that the unfinished tunnel did not appear to be headed toward the stadium, Mr. Wretham said that doesn’t rule out the possibility that the tunnel is connected to someone wanting to disrupt the games. “They could do something horrendous, run to this bunker and possibly go undetected right under the nose of police,” said Mr. Wretham, a former detective sergeant who is currently the co-CEO of Investigative Solutions Network. “I firmly believe that this is for something nefarious,” he said. The size of the tunnel — 10 metres long, 2.5 metres high and 70 centimetres wide — would also make for an ideal place to cache a large quantity of weapons or explosives, Mr. Wretham said.

2. Students from York’s Lassonde School of Engineering

Former Toronto detective Jim Downs said the first place he would look is York’s Lassonde School of Engineering. But York would not answer questions about whether it was looking into possible student hijinks, referring requests to Toronto Police. The engineering school’s website lists a handful of clubs for students — with some devoted to robot fighting and building rovers, but none on tunnelling. Const. Victor Kwong, a Toronto police spokesman, said York confirmed that the tunnel was not part of a school-sanctioned student project.

3. A treasure hunt

It’s unlikely that the burrowers were looking for an artifact of any major value, York University archivist Suzanne Dubeau said Wednesday. “I really doubt it,” she said, noting that before the university was built in 1965, the area was just woodland and farmland. So any allusions to Louis Sacha’s classic 1990s young adult novel Holes are null. “You do know, of course, that the Faculty of Science and Lassonde School of Engineering, they are Canada’s primary research facility into Martian exploration and other space stuff, so I wonder if there’s a connection there,” Ms. Dubeau said with a laugh.

4. An old miner

The tunnel appears to have been constructed in the “cap and leg” method — an outdated way of digging short tunnels by hand that only “an experienced older guy” would use, engineer Gary Benner told the Post on Tuesday. For its part, LIUNA Local 183 — a union that represents professional tunnellers in the Toronto area, said it has “no knowledge of any member being involved in that tunnel.” But, as spokesman Jason Ottey noted, “We have 45,000 members.”