The Guardian:

A pair of amateur explorers in Canada have found a vast underground passage stretching hundreds of metres underneath the bustling streets of Montreal whose formation dates back more than 15,000 years ago to the Earth’s last ice age.

And:

Formed thousands of years ago by massive glaciers that ruptured the rock beneath, yellow calcite line the walls of the passage at times, adding pops of bright colour while icicle-shaped stalactites hang overhead.

I find it incredible that such a massive complex underground tunnel has been hidden for so long. An amazing find.