Secret Ice Age cave system discovered in Montreal.

Cities often have vast underground networks hidden below their surface, but usually

we've put them there to contain our subway trains or power lines or parking garages. In Montreal, Canada, they even hold shopping malls. But the city has a whole new underground space it's exploring: one that was created 15,000 years ago.

Two cavers named Luc Le Blanc and Daniel Caron explored the cave network in October after first becoming suspicious of its existence in 2014. The ancient caves, which stretch about 650 feet, were discovered past a fissure at the end of a cave originally discovered in 1812, located under a city park.

About 3,000 visitors travel to the original cave every year and have done so since the 1980s. But no one had made it past the fissure until Le Blanc and Caron, with their tools, figured out how to pass through it near the cave's ceiling.

Compared to the existing cave it's connected to, the new discovery is "bigger, higher, wider and more beautiful because it's remained pristine," Le Blanc, a caver for 40 years and retired software developer, told Newsweek. "It's the chance of a lifetime to discover a cave like that."

From the entrance Le Blanc and Caron discovered, one direction circles back toward the entrance of the 1812 cave. The other leads down, gradually filling with water. The team used inflatable canoes and swam to explore what they could, but haven't found the end yet. "We stopped when our helmets were rubbing the ceiling," Le Blanc said.

The new cave is a highly unusual type of structure that likely formed about 15,000 years ago, during the last Ice Age through glaciotectonics. First, the weight of ice sheet edges crushes a layer of limestone, then the rock cracks along vertical fissures. "This crushed layer has acted as a kind of lubricant to allow the walls to open," Le Blanc said.

That means the two sides of the cave match, like the coastline of South America nestling into that of Africa, and the top and bottom are flat surfaces.

That's unlike most caves, which are formed by a process called dissolution, in which slightly acidic water eats away at rock as it passes. Le Blanc compares it to a drop of coffee touching a sugar cube: "There's no speed or big movement involved, but the coffee makes its way through the sugar." That's what creates the rough walls we typically associate with caves.

But the walls of the new cave aren't just unusually flat. Because they are made of limestone, they are also full of fossils. And the millennia of water seeping through the cave has created what geologists call flowstones, large white patches of the mineral calcite. Some of these are more than six inches across and represent as much as 20,000 years of geological history, Le Blanc said.

The next step is to find the cave's end, which Le Blanc thinks will be easier in the winter, when the water level should drop. He has brought Jacques Schroeder, a geologist at the University of Quebec at Montreal who has studied glaciotectonic caves elsewhere in Quebec, into the project as well. They also want to use radiolocation to figure out precisely where the caves are in comparison to Montreal's streets.

"The city obviously would want to know which houses are above it," Le Blanc said. "We don't think there's any danger involved."

Culled from Newsweek.