Rescuers raced Monday to free hundreds of workers believed to be trapped in tunnels at hydropower sites that were buried in mud by catastrophic flooding on the Nepal-China border, as the death toll climbed and increasingly desperate families searched for missing loved ones. “These tunnels are big enough to drive trucks through,” said Jakob Steiner, a geoscientist at the University of Graz in Austria based in Dhaka, Bangladesh. “And from what we understand, they have managed to get oxygen to some of those stuck inside and are trying to rescue them now. It’s a race for time, though, as people...