High in the mountains, melting glaciers leave lakes behind. The wall holding one back is often just loose rock and ice. When it fails, the whole lake leaves at once and races down the valley as a wall of water, mud, and boulders.
Warning signs: the river suddenly rises or suddenly stops, on a day with no rain. A roar from upstream. Water turning grey or brown.
What to do: go uphill, away from the river, immediately. Not along the road, roads follow rivers. You may have minutes, not hours.
Nepal's exposure: more than 20 glacial lakes are classified as potentially dangerous, and many more sit across the border in Tibet where Nepal has no monitoring.