The 500m deep Miao Keng shaft, image via the Daily Mail.
The area around the mountain village of Tian Xing seems to have an incredible assortment of caves. It has the four deepest cave systems in China and “numerous 200m+ vertical shafts” including the 491m Miao Keng shaft. A half a kilometre (1600 feet, for the metrically challenged) of sheer vertical drop.
At the bottom of the shaft, the lights are the cavers’ headlamps. Via Daily Mail.
The descent down this shaft takes hours, all performed abseiling on a single rope, and this particular team of cavers spent a week exploring the area. As long as this drop is, it’s not quite the longest in the world: there are several in Croatia and Slovenia that are somewhat longer than it.
Those pictures of the descending cavers are spell-binding. Descending a shaft like that must take some serious nerve, a real act of faith in the strength of your equipment. Would your characters have what it takes to face a thousand foot drop into the unknown? Would they even be able to deal with the logistics of traversing it?