China is racing to evacuate about 80,000 people at risk from large-scale flooding if a swelling lake formed as a result of the 12 May earthquake bursts its banks.
Tangjiashan is the largest of dozens of lakes which have formed behind fragile mud and rubble dams after landslides triggered by the earthquake blocked rivers in the area.
The lake is swelling next to Beichuan, one of the towns hit hardest by the earthquake. On Tuesday water levels were only 26m below the lowest part of the debris barrier, and were still rising.
Emergency workers are aiming to complete the evacuation by midnight on Tuesday, bringing the total number of people relocated from the valley to almost 160,000.
The lake is inaccessible by road. Excavators and bulldozers have been airlifted in by helicopter, while troops have trekked up mountain paths in the devastated region to reach it.
The army has been working continuously to reduce the risks, and is aiming to create a channel through which some of the built-up water can flow out of the lake.
More than 70,000 people in the city of Mianyang have already been moved, with earthquake survivors like these building makeshift shelters on higher ground.
These children played cards by their tent as they waited for the flooding risk to wane.
The Tangjiashan lake now holds 34 billion gallons (130 million cubic metres) of water. Much larger evacuations may be needed, with plans in place to move up to 1.3m people if necessary.
Large parts of the city of Beichuan, 3km (2 miles) from the lake, already lie in ruins. An estimated 80% of buildings in the old town and 60% in the new town have been destroyed.