Pie-XII Park lies in the heart of Montréal, surrounded by busy city streets. But it's what lies beneath those streets that has cave experts and city residents buzzing.

A new network of caves, dating back to the Earth's last Ice Age, and extending nearly 700 feet, were discovered in October by Daniel Caron and Luc Le Blanc, two speleologists (cave experts). The find was only recently announced after the site was secured.

On October 12, Caron and Le Blanc were exploring the already well-known St. Léonard cave that lies just underneath Pie-XII Park. The original portion of the cave had been discovered in 1812, but cave experts have long speculated that more was hidden beneath.

For most of Montréal, however, the large network lurking below the city was unknown.

"They’ve dug sewers and made basements, but no one had ever seen them," La Blanc said of the cave network.

Caron and Le Blanc decided to explore their hunch in 2014, when they began spelunking through the St. Léonard caverns searching for new passages.

Le Blanc, armed with a new radiolocation kit, and Caron, using a divining rod, were hunting for voids or signs of water lying on the other side of the cave's walls. By 2015 they had found a small, narrow opening in the back of one cave. Using a small camera, fellow speleologist François Gelinas was able to peak through the opening, where they saw a large room just behind the wall.

Though eager to bust through the wall, they weren't able to move past it until nearly two years later. The cave walls in St. Léonard are made of solid limestone, and opening a passage required industrial-strength drills.

Related: See inside a 124-mile long cave in China.

Once they were through, they entered a large room, climbed down a big drop, and entered a tall, narrow hall.

"The walls are perfectly smooth and the ceiling is perfectly horizontal," said Le Blanc. They estimate the ceiling to be roughly 20 feet high. In addition to the smooth limestone walls lining the cave, stalagmites and stalactites are found throughout the passage.

According to the Quebec Speleological Society, to which both men belong, a centimeter of stalagmite takes about a thousand years to grow.

Finding caves at high latitudes is less common than when hunting for caves closer to the equator. Most form over time when water dissolves underlying rock, but Quebec's cold temperatures make the water less acidic and thus slow the cave-making process, said Le Blanc.

The St. Léonard caverns were instead formed largely by receding glaciers. During the last Ice Age, which ended about 10,000 years ago, ice would have covered much of North America. In the tens of thousands of years that ice was retreating, rapidly receding glaciers created fissures in the terrain.

"You have evidence of where you have knobs on one side that fit perfectly into a hole on the [opposite] wall," said Le Blanc, describing the walls' parallel faces, like puzzle pieces.

According to Caron, the farthest reaches of the cave eventually reach the Montreal water table, and an aquifer lies beneath.