A mystery bunker found near York University in Toronto has police baffled and many speculating about its possible origins.

Is it a bomb shelter? A storage hole? A prank?

Toronto police say it's not illegal to dig a hole, and they don't think the bunker was meant for any criminal purposes.

"There is nothing to suggest criminality," Deputy Chief Mark Saunders said Tuesday. Saunders says he doesn't have a "working theory" about the bunker. That's why police are asking for the public’s help to solve the mystery

So what was it meant for? Here's what we know.

Where was it?

Toronto police say the bunker is located in a heavily-wooded area some 25.6 metres from the fence line at Rexall Centre, a tennis stadium on the western edge of the York University campus. The bunker's main entrance was concealed under a sheet of metal.

Image released by the Toronto Police Service shows items from the bunker

The Rexall Centre will play host to tennis events for the 2015 Pan American Games this summer. Rexall is also the annual summer site of the Rogers Cup, a professional tennis tournament that draws many top athletes in the sport.

How big is it?

Police say the bunker was 3 metres underground and appeared to have been dug without the use of construction equipment. The tunnel itself was about 1.9 metres high, 0.85 metres wide and 10 metres long.

The tunnel was braced with lumber and plywood.

This image released by Toronto police shows the 10-metre long bunker

"This was built with a considerable amount of sophistication," Saunders said. "The individuals responsible for building it clearly had some expertise in structural integrity."

The tunnel has already been filled in, but police continue to investigate.

The tunnel builders had a professional setup inside, with moisture-resistant lights, an air compressor, a generator and a pump to get rid of water accumulation. Noise-cancelling foam was also found inside.

Image released by the Toronto Police Service shows items from inside of the bunker

In addition to the construction equipment, police recovered a Christian rosary attached to a Remembrance Day poppy. They say it was found hanging on a nail in the bunker.

This rosary with a poppy nailed to it was found inside of the bunker

What exactly is it?

There are many names out there for what police found near York. Some are calling it a bunker. Others, a tunnel. Toronto police have referred to it as a "chamber" and an "underground structure."

Whatever it is, it's definitely an underground hole.

Theories

Queen's University political science professor Christian Leuprecht called it an "inherently disconcerting" discovery, even if police don't suspect terrorism.

"The location might have been purely incidental," he told CTV News Channel on Tuesday. Leuprecht added that the tunnel resembles those used by smugglers near the U.S.-Mexico border.

Security expert Ron Wretham said he doesn't believe the tunnel was made with terrorism in mind.

"I'm not convinced," he told CTV News Channel. He put forward the theory that engineering students may have been behind the tunnel's construction, given the skill required. "Whoever built this had a very good background in engineering and construction."

Twitter speculation ran rampant after news of the tunnel became public. Reactions ranged from serious doomsday speculation to tongue-in-cheek uses for the hole.

Reasons to build a secret tunnel in Toronto: 1 best game of hide and seek 2 apocalypse bunker 3 parties 4 extra storage for sm. apartment — MK (@moonklutz) February 24, 2015

Toronto’s Hottest Club is Mystery Tunnel. It has everything! generators, a poppy, walls and the DJ is a ghost from WWII. #TorontoTunnel — Big Pig Co. (@bigpigco) February 24, 2015

@TraceyKent drug storage, get away tunnel for bank robbers, serial killer hidey hole, so many non-terrorist uses for a secret hole. — ChristinaR (@crobins72ca) February 24, 2015

First though upon hearing about the secret tunnel discovered under my school: Morlocks! — arr (@r_the_iguana) February 24, 2015

Others simply used the mystery to crack jokes at Toronto's expense.

No matter what reason the Toronto secret tunnel at York was dug for, I hope the TTC is looking to hire those folks to get subway done. — tapeworthy (@tapeworthy) February 24, 2015

Secret bunker-like tunnel with lights and generator discovered in Toronto, and three Tim Hortons within walking distance opened immediately. — Pankil Richards (@iMacMe) February 24, 2015