The city's chief of police says those theories included the fact it could have something built for fun.
"One of the theories was maybe this is just the coolest fort ever and it turned out that was probably the closest to being true," Chief Bill Blair said Tuesday, when speaking with reporters the day after police revealed they had closed their investigation into the matter.
Police say they know who built the tunnel and it was simply a case of "two guys who guys who just wanted to dig a cave," as Const. Victor Kwong explained it on Monday.
No charges are being laid.
Blair admits it was an interesting story and one that the public wanted to see investigated.
"It was important I think for us to make sure that we got to the bottom of it and I'm not playing on that pun," Blair said.
"It was a bit of a mystery and a concern and we just wanted to make sure that there was nothing nefarious, no criminal intent behind it."
The police chief said the only risk he sees in the now-filled-in tunnel was that a child could have got inside it.
"It's now been covered over and made safe,” said Blair. “And so mystery solved and resolved."