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The damp, stratified limestone walls and passages are lined with stalactites, formed over thousands of years by tiny drips of water.

Caron and Francois Gelinas, the director of Quebec’s speleological society, led The Canadian Press on a tour of the caves, which are reached by crawling on hands and knees through muddy tunnels, scaling ladders and edging along narrow passages.

The chambers were formed thousands of years ago during the last ice age when the pressure of massive glaciers split the rock beneath the surface, Caron explained.

Photo by Graham Hughes/The Canadian Press

Since the caves are underground and required drills to reach, the pair is sure nobody has ever been inside, with the exception of the small original chamber that has long been open to the public.

“They built the street over the cave and they never found the cave,” Gelinas said, looking up towards the dripping brown ceiling.

While it seems unbelievable that something of this size could exist under a major city, Caron said the undetectable nature of caves is what has inspired him to seek them out for more than 50 years.

Photo by Graham Hughes/The Canadian Press

“Underground excavation is the only thing on the planet where there is no scientific, technical or technological means of knowing if there are caverns, and whether they are large or small,” he said.

Ground penetrating radar doesn’t work, he said, which means the only way to detect caves is through good, old-fashioned human exploration.

Thus far, Caron and his partners have discovered between 250 and 500 metres of caves, although they’re sure the true dimensions are much longer.

Some of the passages require rock-climbing equipment and possibly more rock-breaking to enter.

Another, which is filled clear lake, and has been only partially explored with the help of an inflatable raft.

“Our hope is that when the dry season the water level will lower and we can go further,” he said, adding that divers may also need to be called in.