At least 177 people are missing and 20 have died in northern India after part of a Himalayan glacier fell into a river sending a devastating avalanche of water, dust and rocks down a mountain gorge, and crashing though a dam. Rescue teams worked through the night to find survivors trapped under the debris. Most of the missing are workers from two hydroelectric projects in Uttarakhand"s Chamoli district, which were hit by the avalanche. Footage from Sunday"s disaster shows a fast-moving wall of water and rocks barreling down a narrow gorge and smashing through a dam at the smaller hydroelectric project before surging downstream, wiping out buildings trees and people. Ashok Kumar, Uttarakhand"s police chief, said on Monday that 20 bodies had been recovered in the affected region. He said a further 177 people were still missing, including 35 workers who are believed to be trapped inside a tunnel at the larger, state-owned hydroelectric project. Some 2,500 people in 13 villages were initially cut off by the flash floods sparked by the glacier"s collapse, Ashok Kumar, senior official with the Uttarakhand police said on Monday. Rescuers had reached all 13 villages by Monday afternoon and relief work is now underway in them, Kumar said. Meanwhile, rescue efforts are focused on clearing sludge and debris from the tunnel where the 35 workers are thought to be trapped. Rescuers managed to clear the mouth of the tunnel on Monday, according to a Twitter post from the Uttarakhand State Press Information Bureau. The bureau hailed the "tireless efforts of Indian Army personnel," adding that relief operations in the area were still underway. According to Reuters, teams had managed to drill through 150 meters of the 2.5 km (1.5 miles) long tunnel, but the sheer volume of debris had slowed progress. On Sunday, rescuers pulled 12 people out alive from another, smaller tunnel at the same site, according to Kumar. The ecologically sensitive Himalayan region is prone to flash floods and landslides. Himalayan glaciers are also vulnerable to rising global temperatures because of man-made climate change. As the ice melts, glaciers become unstable and start to retreat. Large glacial lakes can form, and when parts of the glacier in front of it break away they unleash the water trapped behind it causing an outburst of floods. A 2019 study found that Himalayan glaciers are melting twice as fast as last century, losing almost half a meter of ice each year. Others have pointed to a high level of construction along the state"s rivers, which in recent years have seen an increasing number of hydroelectric dams, projects and infrastructure connecting them, such as roads and new developments. While environmentalists have long warned that rampant development in the Himalayan state is an ecological catastrophe waiting to happen, authorities described Sunday"s landslide as a freak event. "This was a one-time incident. The glacier broke, and with ... debris all came down and flooded the power project here," said Kumar, the Uttarakhand police official. Chief Minister Trivendra Singh Rawat said at a press conference Sunday that "Uttarakhand has witnessed a terrible disaster" and the state was expecting "significant loss to human lives and infrastructure." It was after 10 a.m. local time when a piece of the Nanda Devi glacier broke apart, sending torrents of water down the Dhauli Ganga river valley located more than 500 km (310 miles) north of New Delhi, according to authorities. Most of the destruction was centered on two hydropower projects. The Rishiganga Power project — a small dam of 13.2 megawatts — was completely washed away in the deluge, India"s Ministry of Power said in a statement Monday. The state"s chief minister said 35 people were working at the plant when the waters hit, and "roughly 29 to 30 people are missing." Rising waters prompted authorities to issue urgent evacuation notices to people living further down the Alaknanda River. — Courtesy CNN