The valley of Dien Bien Phu is located 250 km from Hanoi, in the highlands close to the border with Laos. It is a basin, 16-km long and 6-km wide, surrounded by hills varying between 400 and 550 metres altitude through which the Nam Yum River crosses. A steep-sided and humid site, Dien Bien Phu is frequently flooded and often blanketed in fog. At one of the crossing points over the Nam Yum is a small village near to which the Japanese rebuilt a landing strip during World War II.