The waterfall is the largest, although not the highest, in the country. The vertical drop is 53m. but it has an impressive 300m span; one end of the falls is in China, the other is in Vietnair. The water volume varies considerably between the dry and rainy seasons: the falls are most impressive from May to September, but swimming during this period in the waterholes below may be difficult due to turbulence.

The falls have three levels, creating a sort of giant staircase, and there's enough water any time, most years, to make the trip worthwhile. Half the pleasure of the visit is walking across paddy fields to reach the base of the falls.

The tails are fed by the Quay Son River. An invisible line halfway across the river marks the border, and rafts (per trip 30,000d) pole out the few metres to exactly the halfway mark - and no further! - from each side. There's been some development of tourist facilities on the Chinese side in recent years, but almost nothing except a bamboo footbridge and a couple of bamboo rafts on the Vietnamese side.

There is no official border checkpoint here, but a police permit is required to visit -you cannot simply rent a motorbike and go there on your own. The permit is officially US$10, but hotels in Cao Bang will do the paperwork for between 100,000d and 200,000d. Let them do it - it's much less hassle than doing it yourself at the Cao Bang police station.

About 10km before the falls, show the permit - and leave your passport - at a roadside checkpoint. An official at the parking area at the falls will then take your permit. You collect each from the same place on return. It was all very straightforward at the time of writing, but be prepared for changes to the regulations.