In a dusty gorge deep in landlocked Laos, work is underway on a project that could change South-East Asia forever.
Despite the protests of countries downstream and the warnings of scientists, a construction company has begun building the first dam across the lower reaches of one of the world's great rivers.
Officially the Lao government says it has not decided whether to go ahead with damming the Mekong at Xayaburi - but all the evidence suggests otherwise.
In April a multi-billion dollar contract was signed for a Thai company, CH Karnchang, to build the dam.
They have not wasted time and at the site newly built roads are busy with trucks, and huge piles of gravel now line the banks of the river.
New homes have been built for people who will have to be resettled.
The Lao government refused the BBC permission to go to Xayaburi to see for ourselves but we were given pictures taken in June by the environmental campaign group, International Rivers.
It had chartered a boat to film and photograph the building work secretly.
"At the dam site we saw dozens of construction vehicles, a large concrete containing wall that had already been built and villagers confirmed that the river had already been widened," said Kirk Herbertson from International Rivers.
"What we see from this is that preparatory construction is now finished and that full construction is now under way."
'Damage'
Laos contends that the work at the site is simply preliminary, in case the dam is approved.
It says all it has done is build roads, construct a camp for workers, and carry out a geological survey on part of the river bed.
"They (the building works) do not under any circumstances affect the Mekong River, which indeed is the lifeblood" of Laos, Viraphonh Viravong from the country's Ministry of Energy and Mines told the BBC.
"Any suggestion that the government of Laos would countenance a misuse of or deliberate harm to the river is, quite simply, absurd."
We showed International Rivers' pictures of the site to Jeremy Carew-Reid, an expert on the Mekong and hydroelectric dams.