A bus plunged into a mountain gorge in the northern Indian state of Uttarakhand on Saturday, killing at least 15 people.The bus was travelling from the national capital, New Delhi, to the town of Pithoragarh when it skidded off the road and into the 500-metre-deep gorge, an official at the police control room for Almora district said.At least 20 people were injured and were being treated at a local hospital, NDTV news channel reported.An average of 135,000 people die on India's roads each year, according to government data.Most accidents are caused by the poor conditions of roads and vehicles, bad driving or overcrowding.