Locals in Thailand were baffled when the water in a canal turned a deep blue-green.

Residents have now pleaded with authorities to investigate the bizarre phenomenon at the Khlong Thai Mueang canal in Samut Prakan province on October 29.

The two-mile-long channel connects two other canals to the Chao Phraya River, a major river in Thailand that flows through Bangkok and plays a vital role in the country's transportation and agriculture.

Villagers living near the canal said there was no foul odour, but reported experiencing skin irritation upon coming into contact with the water. They said they suspected factories had been dumping chemicals into the canal.

Village official Amnart Pothai, 55, said: 'The industrial plants cooperated with us to inspect the location. However, we are still unable to identify the cause. We didn't find any pipes in the canal discharging waste from the factories.'

Resident Wanphen Yimthanom, 60, added: 'The canal water has turned green in the past, but never this strange dark shade. It looks almost like coolant.

'I don't use the water, but I'm worried it might spread to and affect other canals.

Authorities said water samples have been collected and sent to relevant agencies for analysis.