Detian Waterfall belongs to the Chungui River (春归河, Chūnguīhé), which flows between China and Vietnam. The river is only 30m across in this upstream section, which means that people on both sides can see each other going about their business. It's not grand like Niagara Falls, but it's the largest falls spanning two countries in Asia, with the added buzz of being surrounded by karst peaks. The best months to see the waters in their full glory are between July and November.
The falls drop in three stages to create cascades and small pools. Swimming is not allowed, but bamboo rafts (from ¥30) will take you up to the spray. You can also legally cross the Vietnamese border at the 53rd merestone – tourists love doing selfies stepping into what's officially Vietnamese territory.
There is only one direct bus from Nanning’s International Tourism Distribution Centre (¥75, four hours, 7.30am), which stops en route at Langdong bus station (8.30am). The direct bus from the falls back to Nanning leaves at 3pm.
Otherwise you will have to take a bus first to Daxin (大新; from ¥45, 2½ hours, regular departures) from Nanning's Xixiangtang Bus Station, and then switch to a bus headed to Detian (德天; ¥15, two hours, hourly), a further 60km; a cab costs around ¥150.