4. The Tunnel at Niagara Falls, Ontario, Canada

Opened: July 2022

Formed by melting glaciers over 12,000 years ago, Niagara Falls is one of the world’s most iconic natural wonders. But many don’t realise it’s also home to an engineering marvel. Built in 1905, the pioneering Niagara hydropower station generated electricity from the mighty cascades for over a century. And thanks to an ambitious renovation project, you can now see the plant’s fascinating subterranean operations up close. After exploring the main generating floor, take a glass- panelled elevator into the depths of the plant, past six levels of machinery to reach a colossal 670m tunnel, which was painstakingly excavated in 1901 to release spent rapids back into the river. Follow the sound of thundering water, and you’ll be rewarded with unprecedented views of the Horseshoe and American Falls from a new viewing platform on the river’s edge.