An ancient cave system beneath a Montréal park is much more vast than experts believed, the National Post reports.

In 1812, a farmer discovered a cave underneath his property in Montréal’s present-day Saint-Léonard borough. Once used to stockpile ammunition and conceal soldiers during the Rebellions of 1837, the Saint-Léonard cave system in Parc Pie XII is today a tourist attraction and historical landmark. But some speleologists (cave experts) suspected there was more to the natural wonder than met the eye.

Beginning in 2014, two amateur explorers named Daniel Caron and Luc Le Blanc began searching for undiscovered passages in the Saint-Léonard caverns, according to National Geographic. By 2015 they had some leads; in October 2017, they used drills and hammers to break down a cave’s wall to reveal a new cavern.

The stalactite-filled chamber has soaring 20-foot ceilings, and it's connected to a serpentine network of underground tunnels. These passages formed during the Ice Age around 15,000 years ago, when glacier pressure splintered underground rock.

So far, Caron and Le Blanc have explored between 820 to 1640 feet of virgin cave passage, and expect to find even more. They believe the vast network sits atop an aquifer, and ultimately leads to the Montréal water table.

Spelunking the Saint-Léonard cave system is challenging—some passages are filled with water or require special climbing or rock-breaking equipment. The explorers hope that the caves will be easier to investigate during the dry season, and that the receding waters will allow them to reach new depths below Montréal’s surface.

[h/t National Post]