Recent storms have changed the color of seawater at Jomtien Beach due to a plankton bloom.
Water spanning nearly five kilometers turned a dark green as the microscopic animals exploded after storms churned up the sea bottom and the tide changed direction.
The plankton is not hazardous to swimmers.
Beach vendor Siriluck Sripled, 55, said the phenomenon is normal and usually dissipates after two or three days.