Until just a few months ago, no one had ever stepped foot into a network of ice age-era caverns hidden beneath the city of Montreal.

The 15,000-year-old caves were discovered in October by two speleologists or cave experts, reports National Geographic. The discovery was only recently announced after the site had been secured.

Luc Le Blanc and Daniel Caron had been exploring the well-known St. Léonard cave that lies underneath Pie-XII Park when they dug through a wall and discovered a passageway. When they peered through the wall, they saw a large cave with nearly 20-foot-high ceilings. The cave had several passages that branched off, snaking underneath the city overhead.

Caron told The Canadian Press it was every caver's dream to discover a place no one had ever seen before.

"Normally you have to go to the moon to find that kind of thing," he said.

Years in the making

The two men said they had been searching for the network of caves for years. The original cave was discovered in 1812, but cave experts had speculated that there were more hidden that had yet to be found, according to National Geographic.

The pair began exploring the area in 2014. Armed with a dowsing rod (similar to a divination rod) and a radiolocation kit, they looked for any sign of water on any gaps in the walls. Once they found a spot that had potential, they used industrial-strength drills to make their way through the cave's solid limestone walls.

"We started digging in a decomposed layer of limestone that was much softer ... We managed to open a window through which we could see the void beyond," Le Blanc told CBC News.

They made their way through and began exploring as far as they could. They found smooth limestone walls and passages lined with stalagmites and stalactites. They waded through water until they had to deploy an inflatable canoe to navigate areas that were as much as 16 feet deep.

"It keeps going. We haven't reached the end yet," Le Blanc told CBC News.

The lure of the underground

An explorer is inside the original Saint-Leonard cave. (Photo: Société québécoise de spéléologie/Facebook) 

Because the caves are underground and explorers need to use drills to reach them, Le Blanc and Caron are certain no one else has been inside them.

"They built the street over the cave and they never found the cave," François Gelinas, the director of Quebec’s speleological society, told The Canadian Press.

Caron said this hidden aspect of caves is what has motivated him to hunt for them for more than 50 years.

"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.