The Oxus is an unusual river. It has no clear beginning or end. For centuries the river, which is also known as the Amu Darya, has served as the border between Afghanistan and Tajikistan. The Oxus is the longest river in Central Asia, but no one knew where exactly in the mountains it originates. After coursing through 2,400 kilometres, it fizzles out downstream in the Kyzyl Kum desert, never reaching the sea. Amateur British mountaineer Bill Colegrave and his team now say they have finally cracked the age-old mystery of the river's origins. In a recent book Halfway House to Heaven about their expedition to the Afghan Pamirs in 2008, they name a small stream as the Oxus' source. Back in the 19th century, at least four teams of Victorian adventurers competed to identify the river's source. They came up with four different answers, which added a sense of mystery to the river. Expeditions to the area stopped during the turbulent upheavals of the 20th century. The Oxus, which was already so difficult to reach, became further romanticised. Colegrave, who enjoys travelling to remote corners of Central Asia, was mesmerised by the puzzle. When roads opened again in 2005, the avid explorer decided to give it a go. He teamed up with Irish amateur geologist Anthony Kitchin and Afghanistan expert Dillon Coleman. Their expedition approached the region from Tajikistan and proceeded to the upper regions of the Oxus in north-eastern Afghanistan on foot and horseback. Timing was crucial. The best chance of success was in late summer. Paths are blocked by too much water in early summer and ice and snow in winter. The worst part of the journey was crossing a narrow undulating trail with a huge drop on one side, they say. In the Wakhan Corridor, a river valley walled in by mountains, the expedition spent about two weeks examining each of the four contenders once named as the river's source. Colegrave was captivated by the basin's unique flora and fauna, including the rare spiral-horned Marco Polo sheep. 'Normally when you climb mountains, you go higher and higher up, reaching the peaks; but with the Pamirs, you find breathtaking plateaus with a blanket of alpine plants and wild herbs like thyme,' he says. Previous explorers claimed Lake Victoria, Lake Chakmaktin, the Little Pamir River and an ice cave near the border with Pakistan to be the principal source of the Oxus River. Colegrave studied each claim in detail and thought the ice cave was the most probable candidate. 'Where the river starts, there is a big open cave at the bottom of a glacier with water pouring thunderously through,' he says. 'It is already a big river when it comes out of the mountain.' He changed his mind, however, when his team found a stream called the Chelap on the border between Tajikistan and Afghanistan, just above Lake Chakmaktin. The Chelap makes for an unusual geographical phenomenon. It flows down the south side of the Wakhan Range, stands still at a completely flat watershed and splits into a number of channels going west onto the Little Pamir valley and east supplying Lake Chakmaktin. 'The two other possible sources [Little Pamir and Lake Chakmakti] were in fact one together,' he says. Colegrave has now thrown the debate about the river's origin wide open again. The explorer will present his findings at the Royal Geographical Society in London and Hong Kong next year.