There's a 30-foot-wide salt cave beneath the streets of Montreal - Apparently.
Located beneath the historic district of Old Montreal, this cave was formed by the collapse of a salt dome that dates back to the last ice age. The cave's walls are composed of crystallized salt and gypsum, and its ceiling is a mere 10 feet above the city's underground tunnels. The cave was discovered in 2002, and its existence has raised concerns about the stability of the city's infrastructure.