(FORTUNE Magazine) – What really caused the nuclear catastrophe at Chernobyl? What was its human cost? Its impact on world agriculture and nuclear power? In the absence of real knowledge, Western investors assumed the worst. Word of a radioactive cloud over the Ukrainian farmlands caused pandemonium in commodities markets, driving grain and livestock prices up for three successive days. Utilities led a broad stock market retreat. The Dow Jones industrial average fell back below 1800, dropping a record 41.91 points in one day (see Looking Ahead).