New Delhi--A potentially devastating health crisis is quietly unfolding in West Bengal, India, where high levels of arsenic have leached into well water and have put more than a million Indians at risk. The immense scale of the human tragedy has begun to attract the attention of scientists from around the world, who see in this grim event an unprecedented chance to learn more about the health effects of arsenic, a known carcinogen. “We9re talking about numbers [of arsenic-poisoning victims] that have never been seen before in the world,” says University of California, Berkeley, epidemiologist Allan Smith.