For those stranded by a flood, disaster experts say survival depends on access to oxygen, food and water. Catastrophic flooding along the Nepal-China border has killed hundreds and left thousands more missing. On Monday, rescuers in Nepal raced to free hundreds of workers thought to be trapped in hydropower site tunnels buried in mud. Experts say the first 72 hours are critical as the chances of survival drop without access to clean water. Access to breathable air is another key to survival. Suffocation worries grow the longer people are trapped.