High in the Himalayas, above this peaceful valley (in Bhutan) where farmers till a patchwork of emerald-green fields, an icy lake fed by melting glaciers waits to become a "tsunami from the sky."
The lake is swollen dangerously past normal levels, thanks to the global warming that is causing the glaciers to retreat at record speed. But no one knows when the tipping point will come and the lake can take no more, bursting its banks and sending torrents of water crashing into the valley below.
Such floods from above have hit Punakha before, most recently in 1994, a calamity that killed about two dozen people and wiped out livelihoods and homes without warning. But scientists say a new flood could unleash more than twice as much water and be far more catastrophic.