The Gia Lai plateau is built of basalt, laid down by eruptions that ended a few million years ago and left cones and craters scattered across it. Bien Ho occupies three of those craters. Their rims still stand proud of the surrounding tableland, so the lake announces itself from a distance as a low ring of raised ground rather than a dip - you climb slightly to reach a body of water sitting around 800 metres above the sea.

Its oval covers about 228 hectares, and in heavy rain the surface can spread beyond 400. Depth averages between twelve and nineteen metres and has been shrinking as sediment fills the funnels; measurements around 2000 put the mean nearer twenty. What makes the lake genuinely strange is its hydrology. No river runs into it. It sits high on the plateau with nothing feeding it but rain and whatever moves underground, and yet it has never emptied in a dry season, and water leaves continuously through one small outflowing stream.

Vietnamese speakers call it Bien Ho, sea lake, and the Jarai call it T'Nung, which carries the same idea: a sea up in the mountains. Anyone who dismisses that as poetry has not stood on the rim in the wet season. The plateau offers no shelter, wind runs unbroken across 228 hectares of open water, and the lake gets up a genuine chop, with whitecaps rolling into the crater walls and slapping the basalt the way surf works a headland.

The wider names tell you how long people have been trying to describe it. Ho To Nueng, Ho To Nung, Ea Nueng, Ia Nueng - one lake, written down by different ears. It is also the practical thing: the freshwater source for Pleiku and the surrounding communes, which is a heavier responsibility than any of its names suggest.

The Jarai stories about this lake are not scenic. In one, a village stood here, prosperous and loud with gongs, until its cattle began dying. The people hunted deer for a sacrifice to Yang, and while they were still feasting the ground convulsed and dropped the whole settlement into the water. Nobody survived except a couple, Mac May and his wife, who happened to be away visiting relatives and came home to find open water where their village had been.

Another telling centres on two villagers, Ya Pom and Ya Chao, and a white piglet Ya Chao raised on white sand from the shore until it grew as big as a buffalo. The village took it for a sacrifice. Ya Chao refused her share and swore that if she ate it the earth would give way; she fed a piece to a grandchild who would not stop crying, and the earth gave way. A third version is the shortest and the hardest: survivors of an eruption wept for their dead until their tears ran together into streams, and the streams filled the crater.

They are all the same story. Something buried a community here, and the water is what remains of it.

Small pine woods run down to the shore, and the shallows carry lotus, water lily and stands of reed. Kingfishers work the margins; whistling ducks and wild geese dive among the lau reeds; birds known locally as ko tuc and ko vong move through the flowers on the surface. Flocks return here every year, which on a plateau with this many dry months is not a small thing.

Underneath, Bien Ho is one of the Central Highlands' serious fisheries - common carp, grass carp, mud carp, and a run of local species alongside turtles, softshell turtles and eels that have been in the basin for decades. Bahnar and Jarai villages ring the rim, as they have for generations, living off the same water their oldest stories treat as a grave.

Highway 14 brings you out of Pleiku and onto the rim in about seven kilometres, and the lake arrives all at once - no gradual widening of a valley, just raised ground and then water. A statue of the bodhisattva Guanyin stands 15.3 metres tall by the shore, backed by five stone pillars representing the five phases.

Vietnam's culture ministry certified the lake as a national scenic site on 16 November 1988. In September 2014 a panel drawn from the Vietnam Journalists Association and the travel industry put it among the country's five most beautiful natural lakes. Both citations are accurate and both slightly miss the point. What holds you at Bien Ho is not prettiness but the arrangement: a rim you have to climb, water where there should be a summit, and a set of stories about why.