The Santa Susanna Mountains between the San Fernando and Simi Valleys is among the most contaminated places in the United States. For decades during and after World War II, it was used to test rocket fuel and nuclear materials. In 1959, a small nuclear reactor actually melted down there, an incident kept from the public for 20 years. In the meantime, houses were built on the slopes. But, even after they discovered what they were living on, many homeowners did not move away. Now, after generations of local effort, the state and federal governments have agreed to a cleanup. Midnight tonight is the last opportunity for public comment.