Twenty-five years ago this week, in the early hours of Dec. 3, 1984, large amounts of water entered a tank at the Union Carbide factory in Bhopal, India. That water reacted with the 42 tons of methyl isocyanate inside, raising temperature and pressure so high that it began venting massive amounts of gas made up of methyl isocyanate, phosgene, hydrogen cyanide and more. The poisonous cloud swept through neighborhoods near the boundary wall, waking sleeping residents with burning throats and eyes, killing about 4,000 people in the first few hours. Over the next few years, the lingering effects increased that toll to about 15,000 dead, according to government estimates. A quarter-century later thousands of people are still grappling with the effects of the world's worst industrial accident and the continued contamination. Union Carbide was bought by Dow Chemical in 2001, and Dow claims the legal case was resolved in 1989, with responsibility for continued cleanup now falling to the local state government. ( 28 photos total