The World Health Organisation calls it "the largest mass poisoning of a population in history" as a new study shows up to 77 million Bangladeshi's were poisoned by their country's water supply.
The Lancet study shows that in the last decade up to 77 million people in Bangladesh could have been exposed to toxic levels of arsenic in their water supply.
Over a period of ten years researchers studied almost 12,000 people living in one area in the capital Dhaka.
The research found that the arsenic, which is naturally occurring, was the cause of 20% of deaths in the area.
It's thought that the poisoning began in the 1970's when hand pumps were installed to tap the groundwater.
Researchers chose Bangladesh as the subject because for almost 90% of the population groundwater is the main source of their water supply.