Kolkata: Two labourers were killed while cleaning the underground reservoir inside a well-known school in Khardah in North 24-Parganas on Saturday afternoon.The police said the two workers — Pradip Bar and Samir Das — had been engaged to clean the underground reservoir which is being constructed. Preliminary investigation revealsthat both the workers died after inhaling poisonous methane gas.Bar first went inside the reservoir and as he did not come out despite repeated calls from his colleagues, Das went in to find out what had happened to him. Also Read – Heavy rain hits traffic, flightsAs both did not come out, their colleagues informed the police. Police and personnel of the Fire and Emergency Services arrived at the site and brought them out in an unconscious state.They were declared brought dead after they were rushed to hospital.The two deceased hailed from the Sunderbans and Khejuri. The police said the water inside the reservoir had not been cleaned for almost one month.Due to some reaction, methane gas had been produced that led to the death of the two workers.