The deepest sinkhole in the world. Located in Fengjie, China. 30° 45′ 1.8″ N, 109° 28′ 12.36″ E. 
It’s 1.3 mile down and up on stairs. It is not a long walk but very tiring. To me, it is much harder than 10 miles hike in high mountains. 
If you go visit Three Gorges Dam, make sure stop by this place. It is not far from Fengjie, where most tourist boat stops.