KORBA (CHHATTISGARH): An immense expanse of fine grey fly-ash stretches out to meet the distant horizon, its flat surface rippled by slow-spinning whirlwinds. Seen from the road, the National Thermal Power Corporation's (NTPC) ash pond rises up like an enormous ziggurat of expired embers, with a base nearly 1,120 acres wide and a summit rising 40 feet above Dhanras village near the coal town of Korba in Chhattisgarh. The self-described