Machu Picchu, Peru
Why it's controversial: UNESCO warns that the Lost City of the Inca has become "a victim of its own success," with more than 1.2 million visitors in 2015. That figure far exceeds the daily limit of 2,500 agreed to by Peru, and the UN body believes such uncontrolled access means the citadel risks serious erosion that's compounded by each additional footstep.
Instead go to: Choquequirao, which is three-times the size of its more famous "sister city" of Machu Picchu with just a fraction of the tourists (thanks in part to the grueling two-day hike it takes to reach it!).