Explorers in Montreal have discovered a massive new cavern beneath the city.

While people have been exploring the Saint Leonard Cavern since at least 1812, last month’s discovery of a new passage was an unexpected surprise.

“It’s a very rare event in your life [that] you discover so much cave passage,” Luc Le Blanc of the Quebec Caving Society told Global News Canada.

Image zoom Photo: Luc Le Blanc

The newly discovered cave is about 30 feet below a residential neighborhood in Montreal’s east end. It proves that the cave network goes more than 600 feet deeper than previously believed.

In order to investigate the new section, explorers must bring down an inflatable raft and canoe through water that's about 15 feet deep. It’s about 40 degrees Fahrenheit in the caves with 100 percent humidity.

Geologists believe that the cave dates back 15,000 years to the Ice Age. It formed when pressure from a glacier cracked open the limestone beneath current-day Montreal. Over the years, dripping water has formed stalactites from the ceiling of the cave.

City officials are currently investigating the cave to determine how they can preserve the stalactites. They eventually hope to open the new discovery up to the public.